GLTFExporter: Implement KHR_lights_punctual.

[donmccurdy]

Allow export of point, spot, and directional lights to glTF. For best results, lights should have their target as a child with position 0, 0, -1, and be oriented to face in the desired direction — similar to what GLTFLoader generates.

[fernandojsg]

lgtm. I think it could be nice to add spot and point lights on the example for the sake of completeness :)

[mrdoob]

I think it could be nice to add spot and point lights on the example for the sake of completeness :)

Agreed. We can do this in another PR though.

[mrdoob]

Thanks!