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()
It uses locale information to detect default language. It's used by
hb_shape() whenever language is not set on the buffer.
Not sure how to properly test it in the test suite. Tested by observing
that with DejaVu Sans we select the proper local glyph version for U+431
under Serbian locale. See http://www.pango.org/ScriptGallery
Behdad Esfahbod