Reorder combining-class to better suit Arabic shadda mark-mark positioning

As reported by Khaled on the list:

"After the introduction of canonical reordering of combining marks
(commit 34c22f81), I'm no longer able to do mark/mark substitution or
positioning for mark sequences that involve shadda as a first mark (or
most interesting sequences at least).

"After some digging, it turned out that shadda have a ccc=33 while most
Arabic marks that combine with it have a lower ccc value, which results
in the shadda being reordered after the other mark which,
unsurprisingly, breaks my contextual substitution and mkmk anchors."

See:

http://unicode.org/faq/normalization.html#8
http://unicode.org/faq/normalization.html#9

src/hb-ot-shape-private.hh

@@ -98,11 +98,27 @@ is_variation_selector (hb_codepoint_t unicode)
 		   (unicode >= 0xE0100 && unicode <= 0xE01EF));  /* VARIATION SELECTOR-17..256 */
 }
 
+static inline unsigned int
+_hb_unicode_modified_combining_class (hb_unicode_funcs_t *ufuncs,
+				      hb_codepoint_t      unicode)
+{
+  int c = hb_unicode_combining_class (ufuncs, unicode);
+
+  /* Modify the combining-class to suit Arabic better.  See:
+   * http://unicode.org/faq/normalization.html#8
+   * http://unicode.org/faq/normalization.html#9
+   */
+  if (unlikely (hb_in_range<int> (c, 27, 33)))
+    c = c == 33 ? 27 : c + 1;
+
+  return c;
+}
+
 static inline void
 hb_glyph_info_set_unicode_props (hb_glyph_info_t *info, hb_unicode_funcs_t *unicode)
 {
   info->general_category() = hb_unicode_general_category (unicode, info->codepoint);
-  info->combining_class() = hb_unicode_combining_class (unicode, info->codepoint);
+  info->combining_class() = _hb_unicode_modified_combining_class (unicode, info->codepoint);
 }
 
 HB_INTERNAL void _hb_set_unicode_props (hb_buffer_t *buffer);