Issues needing to be solved in WEBGL_video_texture proposal

In #2470 many issues were raised by @msc- @tharkum @zhenyao and others. Opening this issue to capture them separately from that pull request.

In discussions with people working on Chrome Media and color-correct rendering in Chrome, the point has been made that the implicit conversion to an "RGB" color space, which will occur while sampling the video, should define that the result is extended sRGB. This handles wide gamut and HDR videos. Canvases are increasingly color managed in browsers, so the conversion from sRGB to the display color profile will be done by the browser's compositor afterward.

There are many more issues to be discussed. Please add to this discussion and propose pull requests addressing portions of them.

Start by reviewing issue #2470 and the discussion here, especially the requirement that video sampling produce extended sRGB for wide-gamut and HDR content. Identify which other issues need separate proposals or pull requests; done means the concerns are resolved through agreed-upon changes, with portions addressed by proposed pull requests.

Written by the indexing model from the issue text.