Expose GLTFInstance.userData in the API

[literallylara]

Description

It would be useful to have access to custom data embedded in the GLTF file. ThreeJS appears to save this in a userData property, which is already being used in this project for the variant logic. My client uses <model-viewer> to show models that change appearance based on some user input. The userData could be utlized to store information on how the user input needs to be processed for a particular model or material. Another use-case might be to store the annotation data right in the GLTF file.

I would be happy to implement this if this is something that is deemed in-scope for this project.

Version

• model-viewer: v3.5.0