[filter-effects] Color space awareness

[LeaVerou]

Currently, all filters are defined in terms of sRGB-centric SVG filters.
Since the Web is finally moving away from sRGB, we'll eventually need to make them more color space aware, either by defining them differently, or by improving the SVG filters they are based on.

This affects:

• hue-rotate()
• brightness()
• grayscale()
• invert() (@svgeesus?)

[svgeesus]

Yup.

Filter functions must operate in the sRGB color space.
https://drafts.fxtf.org/filter-effects-2/#backdrop-filter-operation

Perhaps the easiest way would be to extend color-interpolation-filters which would make one entire filter use a different colorspace, like display-p3.

Although by default SVG filters operate in linear-light sRGB so extending this to other color spaces would involve creating linear-light versions of p3 etc.

There is also a bizarre and apparently normative note about the (CSS) filter functions:

Note: The color-interpolation-filters property has no affect on Filter Functions, which operate in the sRGB color space.