flatten_medium() should also unify

panda/src/pgraph/nodePath.cxx

@@ -5524,6 +5524,9 @@ flatten_medium() {
   gr.apply_attribs(node());
   int num_removed = gr.flatten(node(), 0);
 
+  gr.collect_vertex_data(node());
+  gr.unify(node());
+
   return num_removed;
 }