Fetches the #hb_direction_t of a script when it is
set horizontally. All right-to-left scripts will return
#HB_DIRECTION_RTL. All left-to-right scripts will return
#HB_DIRECTION_LTR. Scripts that can be written either
horizontally or vertically will return #HB_DIRECTION_INVALID.
Unknown scripts will return #HB_DIRECTION_LTR.
Fetches the #hb_direction_t of a script when it is set horizontally. All right-to-left scripts will return #HB_DIRECTION_RTL. All left-to-right scripts will return #HB_DIRECTION_LTR. Scripts that can be written either horizontally or vertically will return #HB_DIRECTION_INVALID. Unknown scripts will return #HB_DIRECTION_LTR.