al_ustr_prev - Allegro 5 API
bool al_ustr_prev(const ALLEGRO_USTR *us, int *pos)
Find the byte offset of the previous code point in string, before *pos.
*pos does not have to be at the beginning of a code point. Returns true
on success, and the value pointed to by pos will be updated to the
found offset. Otherwise returns false if *pos was already at the
end of the string, and *pos is unmodified.
This function just looks for an appropriate byte; it doesn't check
if found offset is the beginning of a valid code point. If you are working
with possibly invalid UTF-8 strings then it could skip over some invalid