pgraph: sort "off" texture stages when loading from .bam

This fixes an issue where states loaded from .bam file containing more than one "off" texture stage would not compose properly.

Fixes #179

panda/src/pgraph/textureAttrib.cxx

@@ -825,6 +825,7 @@ complete_pointers(TypedWritable **p_list, BamReader *manager) {
     }
   }
   _on_stages.sort();
+  _off_stages.sort();
   _sort_seq = UpdateSeq::old();
   _filtered_seq = UpdateSeq::old();