This is a follow up on #1425 / #1474

IIUC the now introduced GPUSwapChainAlphaMode=opaque hint can have overhead depending on the implementation, as the implementation has e.g. fill alpha values etc. As already pointed out in #1425 (comment), many OSs, including MacOS[1], Android, Wayland[2], X11[3] and maybe Windows allow clients to provide a hint of the form "please composite this assuming the alpha channel is actually all 1, with undefined results if it's not.". These are usually realized either as a region (Wayland/X11) or as single boolean for a surface (MacOS).

Such a opt-in flag should allow to save a canvas size blit on many implementations, by skipping any blending. Most importantly it would move the responsibility to the client: provide content in an easy to optimize way and you become faster.

As this kind of hint has been well established in OS compositors and is available to native clients, I think we should have an equivalent in WebGPU as well.

cc: @kvark, @magcius

1: https://developer.apple.com/documentation/appkit/nsview/1483558-isopaque
2: https://wayland.freedesktop.org/docs/html/apa.html#protocol-spec-wl_surface -> set_opaque_region
3: https://specifications.freedesktop.org/wm-spec/wm-spec-latest.html#idm46291029692400