[sfnt] Add API to get actual paint from `FT_OpaquePaint` (#59703). * src/sfnt/ttcolr.c (tt_face_get_paint): New function to resolve an `FT_OpaquePaint` paint reference into an `FT_COLR_Paint` object of a certain format, which contains the detailed information stored in a paint of the respective format. (read_paint): New function to provide the format specific parsing and to populate the data members of each specific `FT_COLR_Paint` subtype. (read_color_line): New function to parse retrieved color line information into an `FT_ColorLine` object, which has information about the color line extend mode as well as an `FT_ColorStopIterator` object. * src/sfnt/ttcolr.h: Updated.