Add xyY to color sample readouts
This patch adds xyY color space readouts to the Color Picker, Sample Points, etc. It applies to both GIMP-2.99 and GIMP-2.10 and uses babl's xyY color space model: babl#22 (closed)
I've compared the xyY readouts to xicclu readouts for the sRGB color space and for other RGB color spaces, including doing multiple transforms in GIMP from space to space, and including a couple of conversions between linear and "gamma" 32f - the resulting xyY values remain within +/- 0.000001 through all the transforms and match ArgyllCMS "xicclu" readouts within the same +/- 0.000001.
The same "delta" is obtained for RGB values when round-tripping from built-in sRGB through multiple RGB color spaces and back to built-in sRGB, and this happens even without the added xyY code, so these small deltas aren't from the xyY code per se.
I've already used this code to check "Y" values for a target chart, though having XYZ readouts would be even better.
Another use for the xyY color space will be if/when the Change Foreground Color and related tools are given better panels for locating and modifying colors, as the xy plane allows to trace the boundaries of "all real colors" and also to display the color space "triangles" for various RGB working spaces to visually see overlap, chromaticities, etc.