Still not sure about:
1) Case. We pass lowercase for now. Would be nice if graphite was
uppercase 3letter like OpenType,
2) Padding. IMO, tag padding is always with spaces, but Martin was
talking about NUL bytes.
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()
