[truetype] Make `TT_Set_MM_Blend' set named instance index. * src/truetype/ttgxvar.h (GX_Blend): New array `normalized_stylecoords'. * src/truetype/ttgxvar.c (TT_Get_MM_Var): Allocate and fill `normalized_stylecoords'. (TT_Set_MM_Blend): Check instance tuple and adjust `face_index' accordingly.