[sfnt] Add 'COLR' v1 API to iterate color stops (#59703).

* src/sfnt/ttcolr.c (tt_face_get_colorline_stops): New function to
return the current `FT_ColorStop` object from `FT_ColorStopIterator`.
Also increment the iterator.

* src/sfnt/ttcolr.h: Updated.
3 files changed