Well, the world map works properly, and voxels are now being generated correctly.

Cross section at 128 Y: (White = solid voxels)
Changes:
- Got rid of the voxel type, was taking up way too much memory. Each voxel is now just a byte that refers to an entry in the new materials table. (16MB memory usage compared to 1GB)
- Default material (no textures, yet) added.
- Terrain generator fixed and I added a cross section for debugging. (Big) caves have been seen in these cross-sections!
- Terrain generation has been sped up considerably.
- Got rid of the VoxelLayer type, isn't used anymore (layers are pulled apart in an easier way).
- NBT now stores materials.
- Various plugin fixes.