Append a glTF2 3d format from a file, buffer or scene and then write to the filesystem, buffer or scene. Registers this GLTFDocumentExtension instance with GLTFDocument. If [param first_priority] is true, this extension will be ran first. Otherwise, it will be ran 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].