Memory, Performance and Stability Improvements (0.22.0 Alpha)

This update brings improvements, both big and small, but likely not noticeable to the average user.

First of all, I altered the in-memory representation of worlds to use 49% less RAM, while still doing the same tasks.  The new algorithm becomes less memory efficient when there are more than 255 types of blocks and then becomes still more inefficient when there are more than 65,535 types of blocks (a very unlikely circumstance).  Even in the worst case scenario, it should still be using about 25% less RAM than the old way.  Performance should be comparable to the old way, under all circumstances.

Second, I removed some pointless code that was slowing down access to world data just a bit.

Third, I fixed a bug that was causing very irritating crashes under Linux when trying to kill the game's process via Ctrl-C on the console.  This was caused by OpenGL resources being improperly disposed of.