This allows to force a GimpColorArea to display as out-of-gamut color.
Current code was only considering the generic RGB case (outside of [0-1]
range), and in particular not grayscale or indexed images.

Ideally the GimpColorArea widget could be (optionally) made to follow a
context, so that for instance it could update its representation when
the context image changes, or when this image's type changes. Yet since
it is a libgimpwidgets widget, it cannot get such update. Instead I add
a new API function to display the color box with the out-of-gamut
triangle. The decision code for this will have to be done elsewhere.

Use this new API for GimpColorHistory to display non-gray colors in the
history as out-of-gamut on grayscale images, or colors absent of the
palette on indexed images.