Move to C++ backend w/ Blender Python frontend

[snake-biscuits]

C++ mesh converter: https://github.com/snake-biscuits/bsp2gltf

If we can get equivalent quality meshes out of bsp2gltf, imports would get way faster
Then we only need to ship bsp2gltf binaries & feed maps into it

Material imports will likely need to be handled in python though
Entities might also need some Python to process them

We could maybe do some .json conversion in C++ though

I'd also like to be handling trigger brush -> geo conversion in C++