Fixed a draw priority problem.  Made async rendering optional.
Now explicitly tearing down opengl memory in case someone's keeping the context around.  Fixed a circular reference to the scene renderer.  Made more efficient use of the isReady option in the quad tile loader.  Tiles are loading more efficiently. Chunk layer updated to use atlases, but not entirely finished.  Moved quad tile loader over to using an more detailed definition of the images.  Allows for several types of data to be passed in, more to come.