[truetype, snft] Add service methods for `DeltaSetIdxMap` and `VarStore`.

This will be needed for 'COLR' v1 variation support.

* src/truetype/ttgxvar.h (GX_ItemVarData, GX_AxisCoords, GX_VarRegion,
GX_VarItemStore, GX_DeltaSetIdxMap): Move structures to...
* include/freetype/internal/ftmmtypes.h: ... this new file.

* include/freetype/internal/service/svmm.h (MultiMasters): Include
`ftmmtypes.h`.
(FT_Var_Load_Delta_Set_Idx_Map_Func, FT_Var_Load_Item_Var_Store_Func,
FT_Var_Get_Item_Delta_Func, FT_Var_Done_Item_Var_Store_Func,
FT_Var_Done_Delta_Set_Idx_Map_Func): New function typedefs.
(MultiMasters): Add them.
(FT_DEFINE_SERVICE_MULTIMASTERSREC): Updated.

* src/cff/cffdrivr.c (cff_load_item_variation_store,
cff_load_delta_set_index_mapping, cff_get_item_delta,
cff_done_item_variation_store, cff_done_delta_set_index_map): New wrapper
methods calling into mm service.
(cff_service_multi_masters): Updated.

* src/truetype/ttgxvar.c (ft_var_load_item_variation_store,
ft_var_load_delta_set_index_mapping, ft_var_get_item_delta,
ft_var_done_item_variation_store, ft_var_done_delta_set_index_map): Renamed
to ...
(tt_var_load_item_variation_store, tt_var_load_delta_set_index_mapping,
tt_var_get_item_delta, tt_var_done_item_variation_store,
tt_var_done_delta_set_index_map): ... this for consistency.
Mark them as non-static.
* src/truetype/ttgxvar.h: Add corresponding prototypes.

* src/truetype/ttdriver.c (tt_service_gx_multi_masters): Updated.

* src/type1/t1driver.c (t1_service_multi_masters): Updated.
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