[truetype, type1] Add `get_var_blend' to MM service. For internal use; we want to share code between the forthcoming CFF2 support and TrueType. * include/freetype/internal/services/svmm.h (FT_Get_Var_Blend_Func): New typedef. (MultiMasters): Add `get_var_blend'. (FT_Service_MultiMasters): Updated. * src/truetype/ttgxvar.c (tt_get_var_blend): New function. * src/truetype/ttgxvar.h: Updated. * src/truetype/ttdriver.c (tt_service_gx_multi_masters): Updated. * src/type1/t1driver.c (t1_service_multi_masters): Updated.