As for motivation behind the change, I think it makes more sense that way because only the CullVisitor cares about rendering. Say an intersection visitor or update visitor only needs to traverse the subgraph once, not once for each pass. For these visitors there is no point in traversing the subgraph more than once, since it's exactly the same graph. Another motivation is the performance improvement had when an intersection visitor tests against a multipass Technique."
Note about mods by Robert Osfield, "Changed dyanmic_cast<> and check against getTraversalType() to utilize the new asCullVisitor() instead to improve efficiency."
Note, this required adding a unsigned int context ID to the osg::isGLUExtensionSupported(,)
and osg::isGLExtensionSupported(,) functions. This may require reimplementation
of end user code to accomodate the new calling convention.
