Add glTF extension support for Open Brush shaders

[madjin]

Is your feature request related to a problem? Please describe.
Open Brush glb files don't look right in Hubs, this is the sample file I tested and how the glb looks in hubs
https://github.com/madjin/openbrush_tests/blob/main/all_brushes.glb

This is how it should look:

Describe the solution you'd like
Here's the glTF extension with sample code on how to implement: https://github.com/icosa-gallery/three-icosa

It seems that gltfLoader is in this part of the project: https://github.com/mozilla/hubs/blob/master/src/components/gltf-model-plus.js

Describe alternatives you've considered
Not sure, I just really want our drawings to work in Hubs so we can have meetings with them and draw particles in places more easily

Additional context
There is also a $450 bounty for whomever can solve this issue btw

[andybak]

Can something be done to move this forward? The Open Brush team is very interested in supporting this. If we could get someone to work on it, would a PR be likely to be accepted?