Simplify model caching

[lilleyse]

Model.js has a global glTF json cache and a per-context WebGL resources cache. To help simplify the code we might want to consider using the per-context cache for everything. Some other things that could make the code easier to follow and less bug prone would be:

• Avoid saving pipeline extras on the glTF object (CC Crash in GltfPipeline when loading two viewers with models #7688)
• Have a separate file like ModelCache
• Simplify the interaction with 3D Tiles, especially with i3dm tiles.

But mainly I think the area just needs some cleanup, as it probably started out simple but has gotten more complex as glTF 2.0 and 3D Tiles happened.

[j9liu]

@lilleyse can this issue be closed now that ModelExperimental has replaced Model ?