added support in osgUtil::OptimizeStateVisitor for removing duplicate
StateSet's from the scene graph, previously only duplicated StateAttributes
we're removed.
to the StateSet::setGlobalDefault() so that the correct default for the glDepthFunc etc are set up for scene graphs, and allows override of these value which risk of inheriting state on to the rest of the scene graph.
This has allowed the hangglide demo to be simplified since it now doesn't need to set the global Depth instance itself.
