Switching optional references to pointers. Not really much of a difference,
and pointers are more idiomatic and efficient.
Added a World class, which combines a ChunkGenerator and a ChunkGrid and begins paving the way for on the fly chunk generator.
Renamed ChunkGenerator to WorldGenerator, eventually
it will create entities in addition to chunks.
Got rid of WorldGenerator's Border type. This is probably best left to a
much higher level mechanism, and there isn't really much of a need
for it yet.
Added shift key to move camera faster.
Implemented hacky on the fly chunk generation following the player, just
for fun.