Fills in missing fields of p from src in a considered manner.
First, if p does not have direction set, direction is copied from src.
Next, if p and src have the same direction (which can be unset), if p does not have script set, script is copied from src.
Finally, if p and src have the same direction and script (which either can be unset), if p does not have language set, language is copied from src.
#hb_segment_properties_t to fill in.
#hb_segment_properties_t to fill in from.
Fills in missing fields of p from src in a considered manner.
First, if p does not have direction set, direction is copied from src.
Next, if p and src have the same direction (which can be unset), if p does not have script set, script is copied from src.
Finally, if p and src have the same direction and script (which either can be unset), if p does not have language set, language is copied from src.