If a color is not within the indexed image's palette, we can consider it
to be out-of-gamut too (the gamut of such image being its palette).

This is a first step towards fixing #2938. Basically currently opening
indexed images is not made obvious and you can end up thinking GIMP is
broken as when you try to paint with a given FG or BG color, you may get
a completely different color on the canvas. And it is not obvious to
realize why. Now at least, the FG/BG color will tell you when the color
you are trying to paint with is not within the accepted palette.