With this PR we refactor `SceneUtil::KeyframeController` not to require `virtual osg::Callback` inheritance. I suppose such `virtual` overhead is not justified here because it negatively impacts many other classes we derive from `osg::Callback`.
`handle_stateset` is not needed because `UpdateMatrixTransform` is a `NodeCallback` only allowed to be set on a `Node`. `Geode` and `Drawable` do not need explicit logic because they are both derived from `Node`.