Add HB_DIRECTION_IS_FORWARD/BACKWARD

src/hb-shape.c

diff --git a/src/hb-shape.c b/src/hb-shape.c
index 197d4a0..2605af6 100644
--- a/src/hb-shape.c
+++ b/src/hb-shape.c
@@ -130,8 +130,7 @@
 
   /* GPOS / kern */
 
-  /* TODO: Vertical */
-  if (buffer->direction == HB_DIRECTION_RTL)
+  if (HB_DIRECTION_IS_BACKWARD (buffer->direction))
     hb_buffer_reverse (buffer);
 
   buffer->direction = original_direction;