Support SceneKit asset compilation

[keith]

Currently these asset types are just ignored

[keith]

@sebastianv1 are you looking into this one?

[sebastianv1]

Was in the middle of trying to add support and we found that the scnassets were unused and could be deleted.

Some notes from my investigation into the tool though:

• copySceneKitAssets is the tool Xcode invokes. It requires absolute paths, relative paths don't work.
• The tool mostly just copies resources into the target bundle, and invokes scntool on the actual scene file. Here is a helpful attempt at documenting the undocumented scntool https://gist.github.com/matux/4a30c4b4b29d636fe6847b6a3234bc4f

[keith]

cool, thanks for the notes!