[truetype] Actually use metrics variation service. * src/base/ftmm.c: Include FT_SERVICE_METRICS_VARIATIONS_H. (ft_face_get_mvar_service): New auxiliary function to look up metrics variation service. (FT_Set_Var_Design_Coordinates, FT_Set_MM_Blend_Coordinates, FT_Set_Var_Blend_Coordinates): Call metrics variation service. * src/truetype/ttobjs.c (tt_face_init): Use metrics variations for named instances.