This is a trivial class returned by
PartBundleNode::get_bundle(). Its purpose is to hold the actual
PartBundlepointer contained within the
PartBundleNode, so that scene graph flatten operations can safely combine or duplicate
PartBundlesas necessary without affecting high-level bundle operations.
The high-level Actor class defined in direct/src/actor, for instance, will store a list of PartBundleHandles instead of on actual
PartBundles, so that it will be immune to changes from these flatten operations.