WIP: GLTF loading support

[reductor]

Initial WIP for #246

Remaining work

• [ ] Cleanup code (remove dummy prints in example)
• [ ] Make it work

[kolibril13]

Hi there! I wrote a threejs GLTF viewer for jupyter using anywidget that you can find here: https://github.com/kolibril13/anywidget-3dviewer-GLTF