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. With that, 100% is how colors look while you're in SDR mode on many displays