structure of scene graphs. I only considered the case of DEEP_COPY_NODES,
but it is sufficient for me. It is not the most elegant way -- I used
a map to list all the already cloned nodes with multiple parents, to be
looked up whenever such a node is revisited. The good part is, it works."
From Robert Osfield, fixed build problems under Linux 64 due to use of int's to store Node*
in internal maps used in above custom CopyOp. Added ref_ptr<> usage to main to
prevent memory leaks
leaving it up to the Viewer to specify the mode (which by default is MultiThreaded).
Added a check for the presence of osgParticle systems so that threading is
disabled in this case.
