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

As discovered on Chrome Mac:
https://code.google.com/p/chromium/issues/detail?id=452326
diff --git a/src/hb-coretext.cc b/src/hb-coretext.cc
index 4a1e14c..4a45175 100644
--- a/src/hb-coretext.cc
+++ b/src/hb-coretext.cc
@@ -897,6 +897,8 @@
 		   * for this one. */
 		  continue;
 	      }
+	      if (buffer->unicode->is_default_ignorable (ch))
+	        continue;
 
 	      info->codepoint = notdef;
 	      info->cluster = log_clusters[j];