More signedness fixes. As reported by clang 19. * src/autofit/afadjust.c (add_substitute): Make first argument unsigned. Update all callers. Other minor signedness fixes. (af_reverse_character_map_new): Minor signedness fixes. * src/autofit/afgsub.c (af_hash_insert): Minor signedness fixes. * src/autofit/aflatin.c (af_glyph_hints_apply_vertical_separation_adjustments): Make third argument unsigned. Update all callers. Other minor signedness fixes. (af_latin_hints_apply): Minor signedness fixes. * src/bdf/bdflib.c (bdf_parse_bitmap_): Minor signedness fix. * src/truetype/ttobjs.c (tt_size_init_bytecode): Minor signedness fix.