Add two private functions which are currently unused but will
become public once it's clear where exactly we need them:
gimp_color_profile_get_rgb_matrix_colorants(): returns a GimpMatrix2
with the RGB colorants.
gimp_color_profile_new_foobar(): takes an existing profile and creates
a new one with the same RGB colorants but a linear gamma TRC. To be
renamed and made more general.
b8902a7d95