Support scene backgrounds

[Jordan Santell]

Reproducible on model-viewer examples: https://googlewebcomponents.github.io/model-viewer/examples/lighting-and-environment.html

This prevents loading additional scene content AFAICT and results in an inaccurate view of the scene.

5CD06464-8015-40EC-B682-7BD3368967A4 does not appear to be serializable. TypeError: this.background.toJSON is not a function
    at ModelScene.toJSON (three.module.js:25345)
    at Object.serializeEntity (<anonymous>:77:50)
    at EventTarget.__requestEntity (<anonymous>:234:20)
    at <anonymous>:1:15

[Jordan Santell]

Requires landing mrdoob/three.js#16764 and fixing mrdoob/three.js#16763

[Jordan Santell]

Scene backgrounds of render targets no longer break serialization, but it'd be nice to display the texture used for the background.