Alpha property for XRCompositionLayer

[eXponenta]

Should be way to set alpha state to layer same as width/height or transform.

Reasons:

1. Native support for alpha fading for some effects without overheads. Because layers already has alpha blending by content alpha this is not a problem for implementation.

2. Optimisation. Due to that when we set alpha to 0 not needed draw a layer we can use this case for reduce calling updateRenderState for temporary layer disabling.

Alpha fading can be implemented in current version only for non-media layers, because without fading layers can't be used in production code due to bad UX.

For regular layers need to use a middleware for each animation frame - draw content to canvas or framebuffer, apply alpha and upload it to layer, what is catastrophically slow for media layer emulatation when video is too large.

[cabanier]

/agenda allow author to control alpha on a layer

[cabanier]

Looking around other web APIs they seem to favor opacity over alpha.
We already have a blendTextureSourceAlpha attribute so maybe alpha is fine.

[cabanier]

OpenXR also has support for color scaling: https://www.khronos.org/registry/OpenXR/specs/1.0/html/xrspec.html#XR_KHR_composition_layer_color_scale_bias

Would it make sense to allow that in WebXR?

[eXponenta]

Hm, because color scale already apply alpha, in openXR it is right way for implementation.

[eXponenta]

#284