Align on log2 versus linear encoding for CSS, 2D canvas, WebGL, and WebGPU APIs

[ccameron-chromium]

This is discussed here:
https://github.com/ccameron-chromium/ColorWeb-CG/blob/master/hdr-big-picture.md

There exist 4 APIs that need to specify HDR headrooms:

• CSS hdr-color
• 2D Canvas globalHDRHeadroom
• WebGL unpackHDRHeadroom
• WebGPU hdrHeadroom

These must align on using linear or log2 value. If they are to use linear values, then I would suggest using the terms globalLinearHDRHeadroom, unpackLinearHDRHeadroom, and linearHDRHeadroom to ensure no ambiguity.

Moving the cross-specification discussion here. Once there is a resolution, we can update each specification to match it.