Note that this is not complete yet; it only handles advance width
variation.
Activation of the code follows in another commit.
* include/freetype/ftmm.h (FT_Var_Named_Style): Add `psid' member.
* src/truetype/ttgxvar.h (GX_HVarData, GX_AxisCoords, GX_HVarRegion,
GX_HVStore, GX_WidthMap): New auxiliary structures for...
(GX_HVarTable): ... HVAR main structure.
(GX_BlendRec): Add data for HVAR loading.
* src/truetype/ttgxvar.c (FT_FIXED_ONE, FT_fdot14ToFixed,
FT_intToFixed, FT_fixedToInt): New macros.
(ft_var_load_hvar): New function.
(TT_Get_MM_Var): Updated.
(tt_done_blend): Deallocate HVAR data.
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