This solution doesn't store any data in an `FT_GlyphSlot' object.
* include/freetype/freetype.h (FT_LayerIterator): New structure.
(FT_Get_Color_Glyph_Layer): New function.
* include/freetype/internal/sfnt.h (TT_Get_Colr_Layer_Func): New
function type.
(SFNT_Interface, FT_DEFINE_SFNT_INTERFACE): Add it.
* src/base/ftobjs.c (FT_Get_Color_Glyph_Layer): Implement it.
* src/sfnt/ttcolr.c (tt_face_get_colr_layer): New function.
* src/sfnt/ttcolr.h: Updated.
* src/sfnt/sfdriver.c (sfnt_interface): Updated.
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