[sfnt] Add 'COLR' v1 API to retrieve color layers (#59703).

* src/sfnt/ttcolr.c (tt_face_get_paint_layers): New function to get
the layers of a `PaintColrLayers` table in the font, using an
`FT_LayerIterator` from an `FT_PaintColrLayers` object retrieved via
`tt_face_get_paint`.

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