Consider providing an API to override palette color values

FT_Palette_Select allows selection of a palette. CSS allows overriding palette values. As FreeType does not have a way to override entries in its internal palette, in our own code we need to store a parallel palette that we compose out of the font's original palette and the override-specification from CSS which is then used for painting.

It would be useful if FreeType would provide an API for overriding either individual entries of the palette, or allowing configuration of a custom palette that is stored on FT_Face.