[ot] Don't rely on cluster numbers for ensure_native_direction()

src/hb-ot-shape.cc

diff --git a/src/hb-ot-shape.cc b/src/hb-ot-shape.cc
index bbd044f..5ead0b7 100644
--- a/src/hb-ot-shape.cc
+++ b/src/hb-ot-shape.cc
@@ -264,6 +264,7 @@
 static void
 hb_form_clusters (hb_buffer_t *buffer)
 {
+  /* Loop duplicated in hb_ensure_native_direction(). */
   unsigned int base = 0;
   unsigned int count = buffer->len;
   hb_glyph_info_t *info = buffer->info;
@@ -290,7 +291,23 @@
   if ((HB_DIRECTION_IS_HORIZONTAL (direction) && direction != hb_script_get_horizontal_direction (buffer->props.script)) ||
       (HB_DIRECTION_IS_VERTICAL   (direction) && direction != HB_DIRECTION_TTB))
   {
-    hb_buffer_reverse_clusters (buffer);
+    /* Same loop as hb_form_clusters().
+     * Since form_clusters() merged clusters already, we don't merge. */
+    unsigned int base = 0;
+    unsigned int count = buffer->len;
+    hb_glyph_info_t *info = buffer->info;
+    for (unsigned int i = 1; i < count; i++)
+    {
+      if (likely (!HB_UNICODE_GENERAL_CATEGORY_IS_MARK (_hb_glyph_info_get_general_category (&info[i]))))
+      {
+	buffer->reverse_range (base, i);
+	base = i;
+      }
+    }
+    buffer->reverse_range (base, count);
+
+    buffer->reverse ();
+
     buffer->props.direction = HB_DIRECTION_REVERSE (buffer->props.direction);
   }
 }