babl: move to/from xyz and rgbtoxyz matrix functions out of babl.h

babl: move to/from xyz and rgbtoxyz matrix functions out of babl.h

Both of these are useful, but we might want a wider range, permitting
both linear and non-linear variants to be converted to both XYZ and LAB.

The matrix is for now used in babl for the RGB->Y conversion - this might
warrant a more direct API.