Avoid allocating a Lisp_Save_Value in ftfont.c

Avoid allocating a Lisp_Save_Value in ftfont.c

* src/ftfont.c (struct ftfont_cache_data): New member face_refcount.
(ftfont_lookup_cache): Clear it when initializing.
Use make_mint_ptr, since this typically avoids the need to
allocate a Lisp_Save_Value as refcount is now stored elsewhere.
(ftfont_open2, ftfont_close): Manipulate the reference
count in the struct, not in the save object.