[b]Note:[/b] This class is only compiled in editor builds. Run-time glTF loading and saving is [i]not[/i] available in exported projects. References to [GLTFDocument] within a script will cause an error in an exported project. Registers the given [GLTFDocumentExtension] instance with GLTFDocument. If [code]first_priority[/code] is true, this extension will be run first. Otherwise, it will be run last. [b]Note:[/b] Like GLTFDocument itself, all GLTFDocumentExtension classes must be stateless in order to function properly. If you need to store data, use the [code]set_additional_data[/code] and [code]get_additional_data[/code] methods in [GLTFState] or [GLTFNode]. Unregisters all [GLTFDocumentExtension] instances. Unregisters the given [GLTFDocumentExtension] instance.