bug 799869 - [harfbuzz] improve heuristic for choosing between shapers for khmer fonts. r=behdad

gfx/harfbuzz/src/hb-ot-shape-complex-private.hh

@@ -283,16 +283,22 @@ hb_ot_shape_complex_categorize (const hb_ot_shape_planner_t *planner)
       else
 	return &_hb_ot_complex_shaper_default;
 
+
     case HB_SCRIPT_KHMER:
-      /* If the font has 'liga', let the generic shaper do it. */
-      if (!planner->map.found_script[0] ||
+      /* A number of Khmer fonts in the wild don't have a 'pref' feature,
+       * and as such won't shape properly via the Indic shaper;
+       * however, they typically have 'liga' / 'clig' features that implement
+       * the necessary "reordering" by means of ligature substitutions.
+       * So we send such pref-less fonts through the generic shaper instead;
+       * use Indic only if the 'khmr' script has the 'pref' feature. */
+      if (planner->map.found_script[0] &&
 	  hb_ot_layout_language_find_feature (planner->face, HB_OT_TAG_GSUB,
 					      planner->map.script_index[0],
 					      planner->map.language_index[0],
-					      HB_TAG ('l','i','g','a'), NULL))
-	return &_hb_ot_complex_shaper_default;
-      else
+					      HB_TAG ('p','r','e','f'), NULL))
 	return &_hb_ot_complex_shaper_indic;
+      else
+	return &_hb_ot_complex_shaper_default;
 
 
     case HB_SCRIPT_MYANMAR: