Under Vulkan, the WebView render thread is rendering into a more limited Vulkan secondary command buffer. This prevents us from performing certain complex blends and operations which require readback from the framebuffer.

We'd like to detect these cases ahead of time (in DirectRenderer? before?) and promote the entire view hierarchy into a new layer. We can then render to this layer as normal (with complex blending and readback) and finally draw it to Vulkan's secondary command buffer.

This may overlap with some of the overlay refactorings happening for Vulkan / SkiaRenderer.