flatten_strong() should unify too

panda/src/pgraph/nodePath.cxx

@@ -5481,6 +5481,9 @@ flatten_strong() {
   gr.apply_attribs(node());
   int num_removed = gr.flatten(node(), ~0);
 
+  gr.collect_vertex_data(node());
+  gr.unify(node());
+
   return num_removed;
 }