[aat] Reduce unsafe_to_concat calls

src/hb-aat-layout-common.hh

@@ -783,8 +783,6 @@ struct StateTableDriver
     if (!c->in_place)
       buffer->clear_output ();
 
-    buffer->unsafe_to_concat ();
-
     int state = StateTableT::STATE_START_OF_TEXT;
     // If there's only one range, we already checked the flag.
     auto *last_range = ac->range_flags && (ac->range_flags->length > 1) ? &(*ac->range_flags)[0] : nullptr;

src/hb-aat-layout-kerx-table.hh

@@ -869,6 +869,8 @@ struct KerxTable
 
   bool apply (AAT::hb_aat_apply_context_t *c) const
   {
+    c->buffer->unsafe_to_concat ();
+
     typedef typename T::SubTable SubTable;
 
     bool ret = false;

src/hb-aat-layout-morx-table.hh

@@ -1154,6 +1154,9 @@ struct mortmorx
 	      const hb_aat_map_t &map) const
   {
     if (unlikely (!c->buffer->successful)) return;
+
+    c->buffer->unsafe_to_concat ();
+
     c->set_lookup_index (0);
     const Chain<Types> *chain = &firstChain;
     unsigned int count = chainCount;