Add Thai shaper that does SARA AM decomposition / reordering

That's not in the OpenType spec, but it's what MS and Adobe do.
diff --git a/src/hb-ot-shape-complex-private.hh b/src/hb-ot-shape-complex-private.hh
index 7c2c7a9..75948c0 100644
--- a/src/hb-ot-shape-complex-private.hh
+++ b/src/hb-ot-shape-complex-private.hh
@@ -52,6 +52,7 @@
   HB_COMPLEX_SHAPER_IMPLEMENT (arabic) \
   HB_COMPLEX_SHAPER_IMPLEMENT (hangul) \
   HB_COMPLEX_SHAPER_IMPLEMENT (indic) \
+  HB_COMPLEX_SHAPER_IMPLEMENT (thai) \
   /* ^--- Add new shapers here */
 
 enum hb_ot_complex_shaper_t {
@@ -91,6 +92,13 @@
       return hb_ot_complex_shaper_hangul;
 
 
+    /* Unicode-1.1 additions */
+    case HB_SCRIPT_THAI:
+    case HB_SCRIPT_LAO:
+
+      return hb_ot_complex_shaper_thai;
+
+
 
     /* ^--- Add new shapers here */