generate normals when needed

pandatool/src/ptloader/loaderFileTypePandatool.cxx

@@ -119,6 +119,10 @@ load_file(const Filename &path, bool) const {
       egg_data.transform(LMatrix4d::scale_mat(scale));
     }
 
+    if (!egg_data.has_normals()) {
+      egg_data.recompute_polygon_normals();
+    }
+
     result = load_egg_data(egg_data);
   }
   _converter->clear_egg_data();