[Indic] Disallow decomposition of a couple characters

This is a hack for now.  Will be fixed when we do complex-shaper-driven
normalization properly.

The results with or without decomposition are the same, but Uniscribe
does not normalize, so this matches better.

src/hb-unicode.cc

@@ -271,6 +271,9 @@ hb_unicode_decompose (hb_unicode_funcs_t *ufuncs,
   switch (ab) {
     case 0x0AC9  : *a = 0x0AC5; *b= 0x0ABE; return true;
 
+    case 0x0931  : return false;
+    case 0x0B92  : return false;
+
     /* These ones have Unicode decompositions, but we do it
      * this way to be close to what Uniscribe does. */
     case 0x0DDA  : *a = 0x0DD9; *b= 0x0DDA; return true;