When removing default-ignorables, merge clusters

Fixes test-shape, and:
https://code.google.com/p/chromium/issues/detail?id=497578
diff --git a/src/hb-buffer-private.hh b/src/hb-buffer-private.hh
index 6a33962..ced748f 100644
--- a/src/hb-buffer-private.hh
+++ b/src/hb-buffer-private.hh
@@ -192,6 +192,8 @@
 				   unsigned int end);
   HB_INTERNAL void merge_out_clusters (unsigned int start,
 				       unsigned int end);
+  /* Merge clusters for deleting current glyph, and skip it. */
+  HB_INTERNAL void delete_glyph (void);
 
   /* Internal methods */
   HB_INTERNAL bool enlarge (unsigned int size);