1466 - Heightmaps v1		switch from single height map to compound tag of height maps
1467 - Biome Int Array
1484 - Heightmaps v2		switch to a different naming system
1503 - Heightmaps v3		WORLD_SURFACE height map added
1908 - Heightmaps v4		LIGHT_BLOCKING height map removed
1912 - Status j14			chunk status format changed
2203 - Biome length 4096