Can be -1 for NUL-terminated string. This is useful for passing parts
of a larger string to a function without having to copy or modify the
string first.
Affected functions:
hb_tag_t hb_tag_from_string()
hb_direction_from_string()
hb_language_from_string()
hb_script_from_string()
Behdad Esfahbod