layer-browser: Static scenegraph example

[ibgreen]

For #3130

• Using ScenegraphLayer with non-URL scenegraphs appears broken.
• This PR reproduces the issue in the layer browser.

[georgios-uber]

@ibgreen are you still working on this? If not please close.

[ibgreen]

I was thinking about updating this to use the GLTFLoader - no need to mess with the gl context.

Would that be valuable?

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1 out of 2 committers have signed the CLA.

✅ georgios-uber
❌ Ib Green

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