GLTF node class. Represents a GLTF node. GLTF nodes may have names, transforms, children (other GLTF nodes), and more specialized properties (represented by their own classes). https://github.com/KhronosGroup/glTF-Tutorials/blob/master/gltfTutorial/gltfTutorial_004_ScenesNodes.md" Gets additional arbitrary data in this [GLTFNode] instance. This can be used to keep per-node state data in [GLTFDocumentExtension] classes, which is important because they are stateless. The argument should be the [GLTFDocumentExtension] name (does not have to match the extension name in the GLTF file), and the return value can be anything you set. If nothing was set, the return value is null. Sets additional arbitrary data in this [GLTFNode] instance. This can be used to keep per-node state data in [GLTFDocumentExtension] classes, which is important because they are stateless. The first argument should be the [GLTFDocumentExtension] name (does not have to match the extension name in the GLTF file), and the second argument can be anything you want.