Fancy version of a GroupNode that remembers scenegraph structure.
If a block is supplied, a new GroupNode instance is passed to the block and the render root is set to the SceneNode object returned from the block.
gnode should be a GroupNode to use when adding children. root should be the SceneNode object that SceneNode#render is called on to render the ChunkNode.
Calls #<< on the underlying GroupNode.
Find the named ChunkNode.
Add a child ChunkNode.
Calls children on the underlying GroupNode.
Calls delete on the underlying GroupNode.
Remove a ChunkNode.
Calls render on the underlying root SceneNode.
[Validate]