Implement the layer mask blend in Vulkan and WebGPU depthStencil mode.

Start with the named Vulkan and WebGPU pipeline files: source/renderer/vulkan/draw_pipeline_vulkan.cpp and source/renderer/webgpu/render_context_webgpu_impl.cpp. Trace how blend equations and source/destination factors are selected, then read the corresponding render context feature flags. Done means layer masks use the requested blend in depthStencil mode on both backends; verify with the offscreen MSAA=1 scenario described in the issue.

It's a trivial fix:

Implement the layer mask blend in Vulkan and WebGPU depthStencil mode.

- source/renderer/vulkan/draw_pipeline_vulkan.cpp: Vulkan layer mask: build the BlendEquation::layerMask fixed-function blend (src ZERO, dst SRC_ALPHA, op ADD) so depthStencil mode can apply a layer mask like the GL backend. vk_blend_op case.
- source/renderer/vulkan/draw_pipeline_vulkan.cpp: Vulkan layer mask: add vk_src_blend_factor(), returning ZERO for layerMask and ONE otherwise.
- source/renderer/vulkan/draw_pipeline_vulkan.cpp: Vulkan layer mask: vk_dst_blend_factor case returns SRC_ALPHA for layerMask.
- source/renderer/vulkan/draw_pipeline_vulkan.cpp: Vulkan layer mask: feed vk_src_blend_factor() into the color/alpha src blend factors instead of a hardcoded ONE.
- source/renderer/vulkan/render_context_vulkan_impl.cpp: Vulkan layer mask: advertise supportsDepthStencilLayerMask now that the pipeline builder implements the blend, so masks apply on devices without raster ordering (e.g. most mobile GPUs).
- source/renderer/webgpu/render_context_webgpu_impl.cpp: WebGPU layer mask: advertise supportsDepthStencilLayerMask now that makeDrawPipeline builds the blend; YUP forces masked layers into depthStencil mode (atomic mode has no layer-mask op).
- source/renderer/webgpu/render_context_webgpu_impl.cpp: WebGPU layer mask: wgpuBlendOp case returns ADD for layerMask (src ZERO, dst SRC_ALPHA).
- source/renderer/webgpu/render_context_webgpu_impl.cpp: WebGPU layer mask: wgpuDstBlendFactor case returns SRC_ALPHA for layerMask, and add wgpuSrcBlendFactor() returning ZERO for layerMask.
- source/renderer/webgpu/render_context_webgpu_impl.cpp: WebGPU layer mask: feed wgpuSrcBlendFactor() into the blend component src factor instead of a hardcoded ONE.

diff --git a/source/renderer/vulkan/draw_pipeline_vulkan.cpp b/source/renderer/vulkan/draw_pipeline_vulkan.cpp
index d780a6d76d1d91e082681cc86533c0208df81a13..23e616109561f558b19c37f9bb83c3e5b44d5f17 100644
--- a/source/renderer/vulkan/draw_pipeline_vulkan.cpp
+++ b/source/renderer/vulkan/draw_pipeline_vulkan.cpp
@@ -27,10 +27,9 @@ constexpr static VkBlendOp vk_blend_op(gpu::BlendEquation equation)
     case gpu::BlendEquation::max:
       return VK_BLEND_OP_MAX;
     case gpu::BlendEquation::layerMask:
-      // Needs a src factor of ZERO, which this pipeline builder does not
-      // vary yet, so Vulkan leaves supportsDepthStencilLayerMask off and
-      // is never asked for it.
-      break;
+      // dst * srcAlpha: VK_BLEND_FACTOR_ZERO src, VK_BLEND_FACTOR_SRC_ALPHA
+      // dst, added together.
+      return VK_BLEND_OP_ADD;
     case gpu::BlendEquation::screen:
     case gpu::BlendEquation::overlay:
     case gpu::BlendEquation::darken:
@@ -51,6 +50,14 @@ constexpr static VkBlendOp vk_blend_op(gpu::BlendEquation equation)
   RIVE_UNREACHABLE();
 }
 
+constexpr static VkBlendFactor vk_src_blend_factor(gpu::BlendEquation equation)
+{
+  // dst * srcAlpha on the layer-mask path drops the src term entirely; every
+  // other equation this builder emits multiplies src by ONE.
+  return equation == gpu::BlendEquation::layerMask ? VK_BLEND_FACTOR_ZERO
+                                                   : VK_BLEND_FACTOR_ONE;
+}
+
 constexpr static VkBlendFactor vk_dst_blend_factor(gpu::BlendEquation equation)
 {
   switch (equation)
@@ -64,8 +71,8 @@ constexpr static VkBlendFactor vk_dst_blend_factor(gpu::BlendEquation equation)
     case gpu::BlendEquation::max:
       return VK_BLEND_FACTOR_ONE;
     case gpu::BlendEquation::layerMask:
-      // See vk_blend_op().
-      break;
+      // dst * srcAlpha, matching the GL backend's GL_ZERO, GL_SRC_ALPHA.
+      return VK_BLEND_FACTOR_SRC_ALPHA;
     case gpu::BlendEquation::screen:
     case gpu::BlendEquation::overlay:
     case gpu::BlendEquation::darken:
@@ -438,10 +445,10 @@ DrawPipelineVulkan::DrawPipelineVulkan(
           pipelineLayout.renderPassOptions()),
       {
           .blendEnable = blendEquation != gpu::BlendEquation::none,
-          .srcColorBlendFactor = VK_BLEND_FACTOR_ONE,
+          .srcColorBlendFactor = vk_src_blend_factor(blendEquation),
           .dstColorBlendFactor = vk_dst_blend_factor(blendEquation),
           .colorBlendOp = vk_blend_op(blendEquation),
-          .srcAlphaBlendFactor = VK_BLEND_FACTOR_ONE,
+          .srcAlphaBlendFactor = vk_src_blend_factor(blendEquation),
           .dstAlphaBlendFactor = vk_dst_blend_factor(blendEquation),
           .alphaBlendOp = vk_blend_op(blendEquation),
           .colorWriteMask = colorWriteEnabled ? vkutil::kColorWriteMaskRGBA
diff --git a/source/renderer/vulkan/render_context_vulkan_impl.cpp b/source/renderer/vulkan/render_context_vulkan_impl.cpp
index 648f0d1be1da85f66f82e10293d0f3652f7611b6..49ad1c10a80bcd3a2e491c91efbf95b74c3a1c65 100644
--- a/source/renderer/vulkan/render_context_vulkan_impl.cpp
+++ b/source/renderer/vulkan/render_context_vulkan_impl.cpp
@@ -1244,6 +1244,10 @@ RenderContextVulkanImpl::RenderContextVulkanImpl(
 #endif
   m_platformFeatures.supportsClockwiseAtomicMode = m_platformFeatures
                                                        .supportsAtomicMode;
+  // draw_pipeline_vulkan.cpp implements BlendEquation::layerMask, so the
+  // depthStencil path can mask a layer on any device (including those without
+  // raster ordering, e.g. most mobile GPUs).
+  m_platformFeatures.supportsDepthStencilLayerMask = true;
   m_platformFeatures
       .supportsClipPlanes = m_vk->features.shaderClipDistance &&
                             // The Vulkan spec mandates that the minimum value
diff --git a/source/renderer/webgpu/render_context_webgpu_impl.cpp b/source/renderer/webgpu/render_context_webgpu_impl.cpp
index 477741d99461fc30630869cf327c9ae6c0ea06b5..b1e557c3a9c35373bdbb6f679c8359f6cd13627e 100644
--- a/source/renderer/webgpu/render_context_webgpu_impl.cpp
+++ b/source/renderer/webgpu/render_context_webgpu_impl.cpp
@@ -1995,6 +1995,11 @@ RenderContextWebGPUImpl::RenderContextWebGPUImpl(
   }
   m_platformFeatures.atomicPLSInitNeedsDraw = true;
 
+  // makeDrawPipeline implements BlendEquation::layerMask, so the depthStencil
+  // path can mask a layer (YUP forces masked layers into depthStencil mode,
+  // since atomic mode's blend step has no layer-mask op).
+  m_platformFeatures.supportsDepthStencilLayerMask = true;
+
 #ifdef RIVE_WAGYU
   // We can only use advanced blend if the client enabled it when setting up
   // the device.
@@ -3207,10 +3212,9 @@ static WGPUBlendOperation wgpuBlendOp(gpu::BlendEquation blendEquation)
     case gpu::BlendEquation::max:
       return WGPUBlendOperation_Max;
     case gpu::BlendEquation::layerMask:
-      // Needs a src factor of ZERO, which makeDrawPipeline does not vary
-      // yet, so WebGPU leaves supportsDepthStencilLayerMask off and is
-      // never asked for it.
-      RIVE_UNREACHABLE();
+      // dst * srcAlpha: WGPUBlendFactor_Zero src, WGPUBlendFactor_SrcAlpha
+      // dst, added together.
+      return WGPUBlendOperation_Add;
 #ifdef RIVE_WAGYU
     case gpu::BlendEquation::multiply:
       return WGPUBlendOperation_WagyuMultiply;
@@ -3291,12 +3295,21 @@ static WGPUBlendFactor wgpuDstBlendFactor(gpu::BlendEquation blendEquation)
     case gpu::BlendEquation::max:
       return WGPUBlendFactor_One;
     case gpu::BlendEquation::layerMask:
-      // See wgpuBlendOp().
-      RIVE_UNREACHABLE();
+      // dst * srcAlpha, matching the GL backend's GL_ZERO, GL_SRC_ALPHA.
+      return WGPUBlendFactor_SrcAlpha;
   }
   RIVE_UNREACHABLE();
 }
 
+static WGPUBlendFactor wgpuSrcBlendFactor(gpu::BlendEquation blendEquation)
+{
+  // dst * srcAlpha on the layer-mask path drops the src term entirely; every
+  // other equation this builder emits multiplies src by ONE.
+  return blendEquation == gpu::BlendEquation::layerMask
+             ? WGPUBlendFactor_Zero
+             : WGPUBlendFactor_One;
+}
+
 wgpu::RenderPipeline RenderContextWebGPUImpl::makeDrawPipeline(
     gpu::DrawType drawType,
     gpu::ShaderFeatures shaderFeatures,
@@ -3474,7 +3487,7 @@ wgpu::RenderPipeline RenderContextWebGPUImpl::makeDrawPipeline(
 
   const WGPUBlendComponent blendComponent = {
       .operation = wgpuBlendOp(pipelineState.blendEquation),
-      .srcFactor = WGPUBlendFactor_One,
+      .srcFactor = wgpuSrcBlendFactor(pipelineState.blendEquation),
       .dstFactor = wgpuDstBlendFactor(pipelineState.blendEquation),
   };

With this i can create an offscreen context with msaa=1 on atomic modes gpus (WebGPU/Vulkan) and correctly use layer masks.