[subset/hvar/retaingids] Synthesize a DeltaSetIndexMap if needed

Before we were emitting the entire VarStore rows if advMap
was Null. Instead, synthesize an advMap.

20% speed up in subset_glyphs/SourceHanSans-VF.otf/retaingids benchmark.

src/hb-ot-var-hvar-table.hh

@@ -45,7 +45,8 @@ struct index_map_subset_plan_t
   void init (const DeltaSetIndexMap  &index_map,
 	     hb_inc_bimap_t	     &outer_map,
 	     hb_vector_t<hb_set_t *> &inner_sets,
-	     const hb_subset_plan_t  *plan)
+	     const hb_subset_plan_t  *plan,
+	     bool bypass_empty = true)
   {
     map_count = 0;
     outer_bit_count = 0;
@@ -53,11 +54,10 @@ struct index_map_subset_plan_t
     max_inners.init ();
     output_map.init ();
 
-    if (&index_map == &Null (DeltaSetIndexMap)) return;
+    if (bypass_empty && !index_map.get_map_count ()) return;
 
     unsigned int	last_val = (unsigned int)-1;
     hb_codepoint_t	last_gid = HB_CODEPOINT_INVALID;
-    hb_codepoint_t	num_gid = (hb_codepoint_t) hb_min (index_map.get_map_count (), plan->num_output_glyphs ());
 
     outer_bit_count = (index_map.get_width () * 8) - index_map.get_inner_bit_count ();
     max_inners.resize (inner_sets.length);
@@ -68,24 +68,17 @@ struct index_map_subset_plan_t
     unsigned count = new_to_old_gid_list.length;
     for (unsigned j = count; j; j--)
     {
-      hb_codepoint_t gid = new_to_old_gid_list[j - 1].first;
-      if (gid >= num_gid) continue;
-
-      hb_codepoint_t old_gid = new_to_old_gid_list[j - 1].second;
+      hb_codepoint_t gid = new_to_old_gid_list.arrayZ[j - 1].first;
+      hb_codepoint_t old_gid = new_to_old_gid_list.arrayZ[j - 1].second;
 
       unsigned int v = index_map.map (old_gid);
       if (last_gid == HB_CODEPOINT_INVALID)
       {
-	if (gid + 1 != num_gid)
-	{
-	  last_gid = gid + 1;
-	  break;
-	}
 	last_val = v;
 	last_gid = gid;
 	continue;
       }
-      if (v != last_val || gid + 1 != last_gid)
+      if (v != last_val)
 	break;
 
       last_gid = gid;
@@ -120,8 +113,6 @@ struct index_map_subset_plan_t
 	      const hb_vector_t<hb_inc_bimap_t> &inner_maps,
 	      const hb_subset_plan_t *plan)
   {
-    if (input_map == &Null (DeltaSetIndexMap)) return;
-
     for (unsigned int i = 0; i < max_inners.length; i++)
     {
       if (inner_maps[i].get_population () == 0) continue;
@@ -183,7 +174,7 @@ struct hvarvvar_subset_plan_t
     if (unlikely (!index_map_plans.length || !inner_sets.length || !inner_maps.length)) return;
 
     bool retain_adv_map = false;
-    index_map_plans[0].init (*index_maps[0], outer_map, inner_sets, plan);
+    index_map_plans[0].init (*index_maps[0], outer_map, inner_sets, plan, false);
     if (index_maps[0] == &Null (DeltaSetIndexMap))
     {
       retain_adv_map = plan->flags & HB_SUBSET_FLAGS_RETAIN_GIDS;
@@ -200,15 +191,10 @@ struct hvarvvar_subset_plan_t
 
     if (retain_adv_map)
     {
-      hb_codepoint_t next_gid = 0;
       for (const auto &_ : plan->new_to_old_gid_list)
       {
-        hb_codepoint_t gid = _.first;
-
-	inner_maps[0].skip (gid - next_gid);
-
-	inner_maps[0].add (gid);
-	next_gid = gid + 1;
+        hb_codepoint_t old_gid = _.second;
+	inner_maps[0].add (old_gid);
       }
     }
     else