[subset] Fix unecessary trailing 0 bytes left by ContextFormat2 pruning.

Uneeded rules where beind removed from the count by the bytes for them was being left in the font.

src/hb-ot-layout-gsubgpos.hh

@@ -2411,6 +2411,7 @@ struct ContextFormat2_5
     const hb_map_t *lookup_map = c->table_tag == HB_OT_TAG_GSUB ? c->plan->gsub_lookups : c->plan->gpos_lookups;
     bool ret = true;
     int non_zero_index = -1, index = 0;
+    auto snapshot = c->serializer->snapshot();
     for (const auto& _ : + hb_enumerate (ruleSet)
 			 | hb_filter (klass_map, hb_first))
     {
@@ -2422,8 +2423,10 @@ struct ContextFormat2_5
       }
 
       if (coverage_glyph_classes.has (_.first) &&
-	  o->serialize_subset (c, _.second, this, lookup_map, &klass_map))
+	  o->serialize_subset (c, _.second, this, lookup_map, &klass_map)) {
 	non_zero_index = index;
+        snapshot = c->serializer->snapshot();
+      }
 
       index++;
     }
@@ -2437,6 +2440,7 @@ struct ContextFormat2_5
       out->ruleSet.pop ();
       index--;
     }
+    c->serializer->revert (snapshot);
 
     return_trace (bool (out->ruleSet));
   }