Adjust normalizer for out-of-order marks

We are going to implement Unicode Arabic Mark Ordering Algorithm:

  http://www.unicode.org/reports/tr53/tr53-1.pdf

which will reorder marks out of their sorted ccc order.  Adjust
normalizer to stop combining as soon as dangerous ordering is
detected.

src/hb-ot-shape-normalize.cc

@@ -369,46 +369,58 @@ _hb_ot_shape_normalize (const hb_ot_shape_plan_t *plan,
   buffer->clear_output ();
   count = buffer->len;
   unsigned int starter = 0;
+  bool combine = true;
   buffer->next_glyph ();
   while (buffer->idx < count && !buffer->in_error)
   {
     hb_codepoint_t composed, glyph;
-    if (/* We don't try to compose a non-mark character with it's preceding starter.
+    if (combine &&
+	/* We don't try to compose a non-mark character with it's preceding starter.
 	 * This is both an optimization to avoid trying to compose every two neighboring
 	 * glyphs in most scripts AND a desired feature for Hangul.  Apparently Hangul
 	 * fonts are not designed to mix-and-match pre-composed syllables and Jamo. */
-	HB_UNICODE_GENERAL_CATEGORY_IS_MARK (_hb_glyph_info_get_general_category (&buffer->cur())) &&
-	/* If there's anything between the starter and this char, they should have CCC
-	 * smaller than this character's. */
-	(starter == buffer->out_len - 1 ||
-	 _hb_glyph_info_get_modified_combining_class (&buffer->prev()) < _hb_glyph_info_get_modified_combining_class (&buffer->cur())) &&
-	/* And compose. */
-	c.compose (&c,
-		   buffer->out_info[starter].codepoint,
-		   buffer->cur().codepoint,
-		   &composed) &&
-	/* And the font has glyph for the composite. */
-	font->get_nominal_glyph (composed, &glyph))
+	HB_UNICODE_GENERAL_CATEGORY_IS_MARK (_hb_glyph_info_get_general_category (&buffer->cur())))
     {
-      /* Composes. */
-      buffer->next_glyph (); /* Copy to out-buffer. */
-      if (unlikely (buffer->in_error))
-        return;
-      buffer->merge_out_clusters (starter, buffer->out_len);
-      buffer->out_len--; /* Remove the second composable. */
-      /* Modify starter and carry on. */
-      buffer->out_info[starter].codepoint = composed;
-      buffer->out_info[starter].glyph_index() = glyph;
-      _hb_glyph_info_set_unicode_props (&buffer->out_info[starter], buffer);
-
-      continue;
+      if (/* If there's anything between the starter and this char, they should have CCC
+	   * smaller than this character's. */
+	  (starter == buffer->out_len - 1 ||
+	   _hb_glyph_info_get_modified_combining_class (&buffer->prev()) < _hb_glyph_info_get_modified_combining_class (&buffer->cur())) &&
+	  /* And compose. */
+	  c.compose (&c,
+		     buffer->out_info[starter].codepoint,
+		     buffer->cur().codepoint,
+		     &composed) &&
+	  /* And the font has glyph for the composite. */
+	  font->get_nominal_glyph (composed, &glyph))
+      {
+	/* Composes. */
+	buffer->next_glyph (); /* Copy to out-buffer. */
+	if (unlikely (buffer->in_error))
+	  return;
+	buffer->merge_out_clusters (starter, buffer->out_len);
+	buffer->out_len--; /* Remove the second composable. */
+	/* Modify starter and carry on. */
+	buffer->out_info[starter].codepoint = composed;
+	buffer->out_info[starter].glyph_index() = glyph;
+	_hb_glyph_info_set_unicode_props (&buffer->out_info[starter], buffer);
+
+	continue;
+      }
+      else if (/* We sometimes custom-tailor the sorted order of marks. In that case, stop
+		* trying to combine as soon as combining-class drops. */
+	       starter < buffer->out_len - 1 &&
+	       _hb_glyph_info_get_modified_combining_class (&buffer->prev()) > _hb_glyph_info_get_modified_combining_class (&buffer->cur()))
+        combine = false;
     }
 
     /* Blocked, or doesn't compose. */
     buffer->next_glyph ();
 
     if (_hb_glyph_info_get_modified_combining_class (&buffer->prev()) == 0)
+    {
       starter = buffer->out_len - 1;
+      combine = true;
+    }
   }
   buffer->swap_buffers ();