Allow users to bring their own lights

While some content publishers want full control over the scene, there are specifically perf costs for things like shadows and lights. Do we want something like allow-model-lights to import dynamic lights that are found in a glTF model? Would this enable shadows as well? Should this be disabled, even if set, on mobile?

[donmccurdy]

+1 for ignoring lights by default (even though the extension is pretty close to finalized).

Also consider logging a warning when punctual lights are ignored — perhaps even going so far as to say that including lights in an asset is “generally not recommended” or some such thing. I think (but would be willing to discuss) that env maps should similarly be ignored by default.

In general, I am hoping that DCC tool exports and model libraries like Poly will not include lights or environment maps by default, and these defaults can give a slight nudge in that direction.

[elalish]

Done ages ago