Fix some `FT_Fixed` vs. `FT_Long` confusion.

`FT_Fixed` and `FT_Long` are both typedef'ed to be `signed long`.  However,
`FT_Fixed` implies that the lower 16 bits are being used to express
fractional values and so these two types should not be confused.

* include/freetype/internal/services/svmm.h (FT_Set_MM_Blend_Func): Use
`FT_Fixed` for `coords`.  Users are passing `FT_Fixed` and implementations
are taking `FT_Fixed`.
(FT_Get_MM_Blend_Func): Ditto.

* src/autofit/afcjk.c (af_cjk_metrics_check_digits): Use `FT_Long` for
`advance` and `old_advance`.  `advance`'s address is passed as `FT_Long*` to
`af_shaper_get_elem`, which writes the advance in em units (not fixed).  The
exact value is not important here as it is only compared to check whether it
has changed.

* src/autofit/aflatin.c (af_latin_metrics_check_digits): Ditto.