GPU rendering into textures rather than CPU

[daboross]

https://github.com/daboross/screeps-rs/commit/6e730b9aa4e126cfc6567d16f5fd2818ad80d3a4 moves terrain rendering into a CPU-based texture creation followed by passing that texture to the GPU.

I don't fully understand the performance implications of doing this initial texture creation on the CPU, but it seems like it would be good to at least try to use GPU stuff to do this. I mean, it's currently super-fast on my laptop, but ideally this project will target very low-end computers which can't run the regular screeps client.