[truetype] Perform variation store delta computation with 64-bit precision.
* include/freetype/internal/ftmmtypes.h (FT_ItemVarDelta): Make type
explicitly 32-bit.
* include/freetype/internal/services/svmm.h
(FT_Var_Get_Item_Delta_Func): Change return type to `FT_ItemVarDelta`
* truetype/ttgxvar.h (tt_var_get_item_delta): Change return type to
`FT_ItemVarDelta`.
* truetype/ttgxvar.c (tt_var_get_item_delta): Store scalars and deltas
to intermediate array, perform computation using new method
`FT_MulAddFix`.
4 files changed
