A bunch of changes, largely introducing some multi-threading, which is
still not quite fully working.

-Peter Mawhorter
Simplified to 2 threads and put chunk compilation in the main/graphics
thread so that things work now (and they work pretty well).

-Peter Mawhorter
More progress: fixed a bug with terrain generation involving static
variables that weren't being reset properly, and started using
dirt/grass instead of stone for debugging purposes. Next step besides
the optimization work that's now a high priority is cutting into the
strata, which perhaps can be an impetus for implementing simple digging.

-Peter Mawhorter