Until now we only had the case of the CMYK color selector using the
soft-proofing profile. Yet this is also interesting for other color
selectors, in particular to show out-of-gamut colors. Indeed if we
enable soft-proofing on the active image, it is interesting to show the
intersection of the current RGB/Grayscale space gamut with the
soft-proofing gamut.

Right now, this is only used in GimpColorSelect when showing colors in
LCh color space.