Support baked light maps and light map baking from Gaia.

[bryanedds]

From what I understand, implementing this will be quite non-trivial due to the need to write a little software ray-tracer. However, there are some existing implementations we might instead be able to instead plug in -

https://github.com/ErindiGR/LMB

https://github.com/ReubenJCarter/web3d-lightmapper

https://www.embree.org/ (might not support light baking easily out of the box, but perhaps could be adapted to baking a light map for us nonetheless!)

...and I'm sure there's others if we can just find them!

Alternatively, we could try to interface with Blender via command line and a python script to bake the light map for us.

Also, we could maybe try to do the same thing with Godot's baking functionality somehow... - https://docs.godotengine.org/en/3.5/tutorials/3d/baked_lightmaps.html#

Perhaps we could even borrow the portion of Godot's MIT-licensed code that implements the underlying ray-tracing functionality, modularize it enough, and expose it to ourselves...

Overall, I suspect it's possible to add this feature without having to write and maintain our own software ray-tracer.