colorspace: make sdr colorimetry not be about rec.2020 anymore

This was just done because of the wrong assumption that displays needed that
to show the full native gamut. That turned out to be an amdgpu bug though; with
that fixed, most of the 0-100% range is wildly oversaturated.
To make the slider more intuitive, this changes the sdr gamut wideness to instead
interpolate to the native display primaries as indicated by the EDID.

(cherry picked from commit 9c0085f5)