[truetype] Handle 0xFFFF special value inside delta retrieval function

* truetype/ttgxvar.c (tt_hvadvance_adjust, tt_apply_mvar,
tt_var_get_item_delta): Remove special 0xFFFF handling in favor of less
redundant handling inside the tt_var_get_item_delta function, as it is
equivalent to returning a 0 delta. Avoids code-duplication checking for
special value 0xFFFF.
1 file changed