BigChunks and cube-width heightmaps

[CursedFlames]

We want to implement a BigChunk that's effectively a cube-scale chunk, modify heightmaps to be stored on BigChunks and be cube-scale instead, and have regular LevelChunks use wrapper heightmaps that offset coordinates and refer to the cube-scale heightmaps on the BigChunks.

• Support for cube-scale heightmaps in cc-core
• Switch SurfaceTrackerWrapper to be cube-scale, and implement wrapper offset heightmaps
• Handle BigChunk loading and unloading, including heightmap saving
• Make Cubes store references to their containing BigChunk
• Move ColumnCubeMap from chunks onto BigChunk
• Figure out how to make things not horrendously thread-unsafe (see Heightmap thread safety #124)