[buffer] Add sync_so_far()

This removes separate out-buffer, at the cost of possibly changing
idx.

src/hb-buffer.cc

@@ -387,9 +387,11 @@ hb_buffer_t::clear_positions ()
   hb_memset (pos, 0, sizeof (pos[0]) * len);
 }
 
-void
+bool
 hb_buffer_t::sync ()
 {
+  bool ret = false;
+
   assert (have_output);
 
   assert (idx <= len);
@@ -403,12 +405,36 @@ hb_buffer_t::sync ()
     info = out_info;
   }
   len = out_len;
+  ret = true;
 
 reset:
   have_output = false;
   out_len = 0;
   out_info = info;
   idx = 0;
+
+  return ret;
+}
+
+void
+hb_buffer_t::sync_so_far ()
+{
+  bool had_output = have_output;
+  unsigned out_i = out_len;
+  unsigned i = idx;
+
+  if (sync ())
+    idx = out_i;
+  else
+    idx = i;
+
+  if (had_output)
+  {
+    have_output = true;
+    out_len = idx;
+  }
+
+  assert (idx <= len);
 }
 
 bool

src/hb-buffer.hh

@@ -288,7 +288,8 @@ struct hb_buffer_t
 
   HB_INTERNAL void guess_segment_properties ();
 
-  HB_INTERNAL void sync ();
+  HB_INTERNAL bool sync ();
+  HB_INTERNAL void sync_so_far ();
   HB_INTERNAL void clear_output ();
   HB_INTERNAL void clear_positions ();