Use ptrdiff_t for composition IDs.

Use ptrdiff_t for composition IDs.

* character.c (lisp_string_width):
* composite.c (composition_table_size, n_compositions)
(get_composition_id, composition_gstring_from_id):
* dispextern.h (struct glyph_string.cmp_id, struct composition_it.id):
* xdisp.c (BUILD_COMPOSITE_GLYPH_STRING):
* window.c (Frecenter):
Use ptrdiff_t, not int, for composition IDs.
* composite.c (get_composition_id): Check for integer overflow.
* composite.h: Adjust prototypes to match the above changes.