backends/drm: add brightness metadata overrides and sdr gamut wideness setting

The brightness overrides are for displays with missing or broken brightness
data in their EDID, and allow the user to work around those displays. In
the future we could also offer an HDR calibration process that allows determining
the correct brightness values for the screen.

The gamut wideness setting allows the user to tweak what gamut KWin assumes
sRGB applications to have. This is useful for working around the gamut mapping
displays do, which make sRGB content look washed out, and also to allow
users to make colors of sRGB apps look more saturated if they wish to.