[subset] shrink the serialize buffer when pruning empty offsets in ChainContextFormat2.

Currently the code reduces the array length, but does not trim back the space allocated in the serializer for those empty offsets.

src/hb-ot-layout-gsubgpos.hh

@@ -2794,9 +2794,10 @@ struct ChainContextFormat2
     if (unlikely (!c->serializer->check_success (!lookahead_klass_map.in_error ())))
       return_trace (false);
 
-    unsigned non_zero_index = 0, index = 0;
+    int non_zero_index = -1, index = 0;
     bool ret = true;
     const hb_map_t *lookup_map = c->table_tag == HB_OT_TAG_GSUB ? c->plan->gsub_lookups : c->plan->gpos_lookups;
+    auto last_non_zero = c->serializer->snapshot ();
     for (const OffsetTo<ChainRuleSet>& _ : + hb_enumerate (ruleSet)
 					   | hb_filter (input_klass_map, hb_first)
 					   | hb_map (hb_second))
@@ -2812,19 +2813,20 @@ struct ChainContextFormat2
 			       &backtrack_klass_map,
 			       &input_klass_map,
 			       &lookahead_klass_map))
+      {
+        last_non_zero = c->serializer->snapshot ();
 	non_zero_index = index;
+      }
 
       index++;
     }
 
     if (!ret) return_trace (ret);
 
-    //prune empty trailing ruleSets
-    --index;
-    while (index > non_zero_index)
-    {
-      out->ruleSet.pop ();
-      index--;
+    // prune empty trailing ruleSets
+    if (index > non_zero_index) {
+      c->serializer->revert (last_non_zero);
+      out->ruleSet.len = non_zero_index + 1;
     }
 
     return_trace (bool (out->ruleSet));