[FYI, not ready for review] Add globalHDRHeadroom to CanvasCompositing interface

[ccameron-chromium]

Add globalHDRHeadroom to CanvasCompositing interface.

Fixes #11165

• At least two implementers are interested (and none opposed):
  • Mozilla: CanvasCompositing globalHDRHeadroom attribute mozilla/standards-positions#1290
  • WebKit: CanvasCompositing globalHDRHeadroom attribute WebKit/standards-positions#542
  • Chromium: positive
• Tests are written and can be reviewed and commented upon at:
  • see tests here
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/428357629
  • Gecko: …
  • WebKit: …
• Corresponding HTML AAM & ARIA in HTML issues & PRs:
• MDN issue is filed: …

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/canvas.html ( diff )
/infrastructure.html ( diff )
/references.html ( diff )