Limit SubChunk to 2 block layers

[dktapps]

Problem description

Currently we allow an arbitrary number of block layers in SubChunk. This impacts performance because of an extra level of indirection (deref property + array lookup, instead of just deref property).

Minecraft itself only permits the use of 2 layers anyway (one for blocks, one for water), so it doesn't really make sense to nerf performance for this.

Proposed solution

Get rid of array $blockLayers and just have two PalettedBlockArray properties for layer 0 and layer 1.

Side note: It's kinda annoying to name these. We can't really call layer 1 the liquid layer since liquids normally appear in layer 0.

Alternative solutions that don't require API changes