GLTFLoader: Pass unknown extensions through as .userData

[donmccurdy]

glTF 2.0 allows creation of custom extensions, which may be widely supported or specific to a single application. We won't implement all of these in THREE.GLTFLoader, but should provide enough flexibility for users to simple self-service extensions.

An easy place to start: when an object contains extensions not recognized by GLTFLoader, the extension JSON should be passed through verbatim as node.userData wherever possible. As examples, see EXT_animation_map and BLENDER_physics.

Related: #11682

[takahirox]

Good to enable users to handle extensions GLTFLoader can't recognize by themselves.

One concern. Sometimes or often parameters in extension can have references to meshes, textures, accessors, so on. So will we return all parsed data to users?

[donmccurdy]

Maybe... In cases like Draco it is very risk to request buffers before they have been used by a known object in the scene. I think we could start by just returning the JSON.

But later on to support extensions that depend on buffers or meshes, maybe we do this:

loader.load('foo.gltf', (gltf) => {
  var scene = gltf.scene;
  var fooExtension = scene.extensions.EXT_foo;;
  var fooBuffer = await gltf.parser.getDependency( 'bufferView', fooExtension.bufferView );
});

[takahirox]

I think we could start by just returning the JSON.

I haven't seen the links above but some extensions can be handled if we just return JSON? If so, yeah it'd be good start.