[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 *));
 };
 
 

src/hb-buffer.cc

diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index 1709305..420da82 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -1678,3 +1678,24 @@
     }
   normalize_glyphs_cluster (buffer, start, end, backward);
 }
+
+void
+hb_buffer_t::sort (unsigned int start, unsigned int end, int(*compar)(const hb_glyph_info_t *, const hb_glyph_info_t *))
+{
+  assert (!have_positions);
+  for (unsigned int i = start + 1; i < end; i++)
+  {
+    unsigned int j = i;
+    while (j > start && compar (&info[j - 1], &info[i]) > 0)
+      j--;
+    if (i == j)
+      continue;
+    /* Move item i to occupy place for item j, shift what's in between. */
+    merge_clusters (j, i + 1);
+    {
+      hb_glyph_info_t t = info[i];
+      memmove (&info[j + 1], &info[j], (i - j) * sizeof (hb_glyph_info_t));
+      info[j] = t;
+    }
+  }
+}

src/hb-ot-shape-normalize.cc

diff --git a/src/hb-ot-shape-normalize.cc b/src/hb-ot-shape-normalize.cc
index dff7a74..a706461 100644
--- a/src/hb-ot-shape-normalize.cc
+++ b/src/hb-ot-shape-normalize.cc
@@ -350,7 +350,7 @@
       continue;
     }
 
-    hb_stable_sort (buffer->info + i, end - i, compare_combining_class);
+    buffer->sort (i, end, compare_combining_class);
 
     i = end;
   }