[tibetan] Reorder marks to better match Uniscribe

Fixes https://github.com/behdad/harfbuzz/issues/237

Note that Uniscribe enforces a certain syllable order.  We don't.
But with this change, I get all of the tibetan contractions pass
with Microsoft Himalaya font.

src/hb-unicode-private.hh

diff --git a/src/hb-unicode-private.hh b/src/hb-unicode-private.hh
index ed45374..0cb99f0 100644
--- a/src/hb-unicode-private.hh
+++ b/src/hb-unicode-private.hh
@@ -115,6 +115,8 @@
     /* XXX This hack belongs to the Tibetan shaper:
      * Reorder PADMA to ensure it comes after any vowel marks. */
     if (unlikely (unicode == 0x0FC6u)) return 254;
+    /* Reorder TSA -PHRU to reorder before U+0F74 */
+    if (unlikely (unicode == 0x0F39u)) return 127;
 
     return _hb_modified_combining_class[combining_class (unicode)];
   }
@@ -343,10 +345,12 @@
 #define HB_MODIFIED_COMBINING_CLASS_CCC118 118 /* sign u / sign uu */
 #define HB_MODIFIED_COMBINING_CLASS_CCC122 122 /* mai * */
 
-/* Tibetan */
+/* Tibetan
+ * Modify U+0F74 (ccc=132) to reorder before ccc=130 marks.
+ */
 #define HB_MODIFIED_COMBINING_CLASS_CCC129 129 /* sign aa */
 #define HB_MODIFIED_COMBINING_CLASS_CCC130 130 /* sign i */
-#define HB_MODIFIED_COMBINING_CLASS_CCC132 132 /* sign u */
+#define HB_MODIFIED_COMBINING_CLASS_CCC132 128 /* sign u */
 
 
 /* Misc */