For multiple master fonts, common usage (in Postscript) is to modify
the WeightVector of an existing font instance, this addition
supports that use.
* include/freetype/ftmm.h, src/base/ftmm.c (FT_Set_MM_WeightVector,
FT_Get_MM_WeightVector): New API functions.
* include/freetype/internalservices/svmm.h
(FT_Set_MM_WeightVector_Func, FT_Get_MM_WeightVector_Func): New
function types.
(MultiMasters): Add `set_mm_weightvector' and `get_mm_weightvector'
members.
(FT_DEFINE_SERVICE_MULTIMASTERSREC): Updated.
* src/cffcffdrivr.c (cff_set_mm_weightvector,
cff_get_mm_weightvector): New functions.
(cff_service_multi_masters): Register them.
* src/truetype/ttdriver.c (tt_service_gx_multi_masters): Updated.
This driver doesn't use the new interface.
* src/type1/t1load.c (T1_Set_MM_WeightVector,
T1_Get_MM_WeightVector): New functions.
* src/type1/t1driver.c (t1_service_multi_masters): Register them.
* src/type1/t1load.h: Updated.
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