Add drawingBufferColorSpace/unpackColorSpace attributes to WebGLRenderingContextBase.

[kenrussell]

Define their behavior both for presenting the rendering context's
results, and uploading textures via texImage2D / texSubImage2D from
DOM inputs (the TexImageSource union type).

Follows whatwg/html#6562 , in which the
PredefinedColorSpace enum was defined, and its behavior for 2D canvas
contexts specified.

[kenrussell]

@jdashg @ccameron-chromium @RafaelCintron in particular (active participants in the ColorWeb CG), please take a look.

[kdashg]

For ease of use, we should consider taking a context creation attrib for colorSpace, and filling these attribs in from that initial value. We should probably exclude it from getContextAttributes, though, since those are immutable results.

[ccameron-chromium]

For ease of use, we should consider taking a context creation attrib for colorSpace, and filling these attribs in from that initial value. We should probably exclude it from getContextAttributes, though, since those are immutable results.

That SGTM.

[kenrussell]

For ease of use, we should consider taking a context creation attrib for colorSpace, and filling these attribs in from that initial value. We should probably exclude it from getContextAttributes, though, since those are immutable results.

That SGTM.

@kdashg @ccameron-chromium sounds fine; let's do that in a follow-up since we have agreement on the rest of this PR at the moment.

[kenrussell]

All conversations resolved, and approved - squashing and merging.