[sfnt] Correct FT_ColorStopIterator documentation.

* include/freetype/ftcolor.h (FT_ColorStopIterator): the user should not
 set `p` to NULL before calling `FT_Get_Colorline_Stops`. `p` and
`num_color_stops` are set by `FT_Get_Paint`. `p` and
`current_color_stop` are updated by `FT_Get_Colorline_Stops`.
1 file changed