to allow it to use different comparison schemes, now it can use checking for pointer equality (the default)
or attribute contexts. This has been added to assist optimization of databases within the flt loader, but
also could be useful for other operations too.
these global attributes are created by cloning any attributes which are
applied during rendering, the clone in a shallow copy, which will set up
default valus for that attribute. This should prevent attribute bleed
from one stateset to the next when the global StateSet doesn't contain
an attribute used within the scene graph.
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.
