Alpha should apply to layer as a whole, not every element in layer

When modifying the with_winit example to render with a non-unity alpha, such as

renderer.render(lottie, frame, Affine::IDENTITY, 0.5)

the result looks like with each component of the tiger having alpha applied to it individually. Instead, the alpha should apply to the rendered image as a whole.

I suspect, but haven't confirmed, that the same issue exists for layers specified by the lottie file.