[coretext] Don't generate notdef glyph for default-ignorables

As discovered on Chrome Mac:
https://code.google.com/p/chromium/issues/detail?id=452326

src/hb-coretext.cc

@@ -897,6 +897,8 @@ retry:
 		   * for this one. */
 		  continue;
 	      }
+	      if (buffer->unicode->is_default_ignorable (ch))
+	        continue;
 
 	      info->codepoint = notdef;
 	      info->cluster = log_clusters[j];

src/hb-ot-shape.cc

@@ -676,7 +676,7 @@ hb_ot_hide_default_ignorables (hb_ot_shape_context_t *c)
 	pos[i].y_advance = 0;
       }
       else
-	continue; /* Delete it. */
+	continue; /* Delete it. XXX Merge clusters? */
     }
     if (j != i)
     {