[OT] Merge clusters when reordering marks for normalization

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=541608
and cluster test.

src/hb-buffer-private.hh

diff --git a/src/hb-buffer-private.hh b/src/hb-buffer-private.hh
index 9aa5e7d..7fed738 100644
--- a/src/hb-buffer-private.hh
+++ b/src/hb-buffer-private.hh
@@ -201,6 +201,8 @@
   HB_INTERNAL scratch_buffer_t *get_scratch_buffer (unsigned int *size);
 
   inline void clear_context (unsigned int side) { context_len[side] = 0; }
+
+  HB_INTERNAL void sort (unsigned int start, unsigned int end, int(*compar)(const hb_glyph_info_t *, const hb_glyph_info_t *));
 };