bam2egg: retain model nodes

added code to retain model nodes that are not DCS nodes.

panda/src/egg2pg/eggSaver.cxx

@@ -940,6 +940,9 @@ apply_node_properties(EggGroup *egg_group, PandaNode *node, bool allow_backstage
     ModelNode *model_node = DCAST(ModelNode, node);
     switch (model_node->get_preserve_transform()) {
     case ModelNode::PT_none:
+      egg_group->set_model_flag(true);
+      break;
+
     case ModelNode::PT_drop_node:
       break;