This PR removes dummy serialisers for `StateSetUpdater`, `NodeCallback` and the respective `META` macros that trigger serialisation requirement here.
`StateSetUpdater` and `NodeCallback` are just base classes that can not be used on their own, so there is no need to incorporate them into serialisation. These changes might have minor effects on derived classes that forget to override `className()`, `libraryName()` through `META`, but it makes hardly a difference to now serialise such classes as a dysfunctional `osg::Callback` instead of a dysfunctional `SceneUtil::NodeCallback`.
Properly apply transformations to both switch and LOD nodes
Allow both NiSwitchNode and NiLODNode to be the root node
Properly add CollisionSwitch into the scene graph
Double buffering, custom bounding box and the update in the cull visitor (instead of update) are now all handled internally rather than needing hacks and/or callbacks.
The double buffering is an implementation detail so it should be handled as such, rather than mandating the scene graph to be structured in a certain way.
Override accept(NodeVisitor&) instead of using callbacks.
