babl-space: babl_space_new_from_chromaticities add equalize_matrix argument

babl-space: babl_space_new_from_chromaticities add equalize_matrix argument

Part of the problem in issue #18

This API is internal only for now so we can change it, changing to use a
hard-coded pre-quantized chromaticities set for the internal sRGB space.

This preserve the gray axis matrix equalization performed for other spaces used
with babl, ideally we can improve the gray-axis equalization to match and
return to using it also for sRGB.