Proposal: Turn on GL backend for Femtovg WGPU

Feature Description

Currently OpenGL backend explicitly disabled for Femtovg WGPU renderer:

            i_slint_core::graphics::wgpu_29::init_instance_adapter_device_queue_surface(
                surface_target,
                requested_graphics_api,
                /* rendering artifacts :( */
                wgpu::Backends::GL,
            )?;

So I made a fork, where enable OpenGL backend for Femtovg WGPU and tested it for the next examples

1. gallery
2. memory
3. slide_puzzle
4. system-tray
5. dnd-kanban
6. imagefilter
7. runtime_key_bindings
8. speedometer
9. carousel
10. todo
11. todo-mvc
12. virtual_keyboard
13. 7guis
14. wgpu_texture

I tested both on Windows and Linux, have not seen any artifacts or any other kind of bug

Linux env: Intel (iGPU, native Mesa) Mesa Intel(R) Graphics (RPL-S) OpenGL 4.6 (Core/Compat) Mesa 26.1.2, OpenGL ES 3.2 Mesa 26.1.2 NVIDIA (RTX 4070, via zink) │ zink Vulkan 1.4(NVIDIA GeForce RTX 4070 Laptop GPU (NVIDIA_PROPRIETARY)) Tested on both chips

Windows env: AMD Radeon(TM) 610M from ATI Technologies Inc. OpenGL version 4.6.0 Compatibility Profile Context 24.30.34.07.250530

Slint version: 1.17.0 (68c8ced) Cargo features: "renderer-femtovg-wgpu", "unstable-wgpu-29", "backend-winit" Tested with env WGPU_BACKEND="gl"

Enabling OpenGL will increase Slint compatibility a lot in my opinion

Product Impact

No response