Track down memory leaks on RenderWidget exit

If you run the visualizer under valgrind, you see that the RenderWidget currently leaks a lot of memory. Most of that is cleanup down in the ign-rendering layer. There was an earlier attempt to cleanup some of that memory here: https://github.com/ToyotaResearchInstitute/delphyne/blob/master/visualizer/RenderWidget.cc#L169 , but that seems to cause a hang somewhere down in the ogre library. To really get rid of the memory leaks, we should do the following:

1. Re-enable the cleanup call pointed out above, and track down the source of the hang.
2. Find out where the other leaking memory is going to. From my brief look at it, a bunch of it was because there were a lot of references to shared_ptr's within ign-rendering that were never released, so we probably need to figure out those references and when to release them.

Start by running the visualizer under Valgrind and inspect the cleanup call at visualizer/RenderWidget.cc line 169. Re-enable that call, investigate the resulting hang in the Ogre library, then trace remaining ign-rendering shared_ptr references. Done means the hang is resolved and the reported memory leaks are cleaned up.