[subset/closure] Batch recursions in scheduled stages

Going to revert. Doesn't pass tests and savings are minor.

src/hb-ot-layout-gsub-table.hh

@@ -53,9 +53,14 @@ template <typename context_t>
 /*static*/ typename hb_closure_context_t::return_t SubstLookup::closure_glyphs_recurse_func (hb_closure_context_t *c, unsigned lookup_index, hb_set_t *covered_seq_indices, unsigned seq_index, unsigned end_index)
 {
   const SubstLookup &l = c->face->table.GSUB.get_relaxed ()->table->get_lookup (lookup_index);
-  if (l.may_have_non_1to1 ())
+  if (covered_seq_indices && l.may_have_non_1to1 ())
       hb_set_add_range (covered_seq_indices, seq_index, end_index);
-  return l.dispatch (c);
+
+  if (covered_seq_indices)
+    c->schedule (lookup_index);
+  else
+    l.dispatch (c);
+  return hb_empty_t ();
 }
 
 template <>

src/hb-ot-layout-gsubgpos.hh

@@ -183,8 +183,22 @@ struct hb_closure_context_t :
 
   void set_recurse_func (recurse_func_t func) { recurse_func = func; }
 
+  void schedule (unsigned lookup_index)
+  {
+    scheduled.add (lookup_index);
+  }
+
   void flush ()
   {
+    hb_set_t scheduled_copy;
+    while (scheduled)
+    {
+      scheduled_copy = scheduled;
+      scheduled.clear ();
+      for (unsigned lookup_index : scheduled_copy)
+	recurse (lookup_index, nullptr, 0, 0);
+    }
+
     output->del_range (face->get_num_glyphs (), HB_SET_VALUE_INVALID);	/* Remove invalid glyphs. */
     glyphs->union_ (*output);
     output->clear ();
@@ -196,6 +210,7 @@ struct hb_closure_context_t :
   hb_map_t *done_lookups_glyph_count;
   hb_hashmap_t<unsigned, hb::unique_ptr<hb_set_t>> *done_lookups_glyph_set;
   unsigned int lookup_count = 0;
+  hb_set_t scheduled;
 };