Change Composable generator pipeline - Shape & Composition

[madmaxoft]

Current generator is suited only for heightmap-based generators - the overhangs are quite difficult to implement and non-conceptual.

Instead of having a terrain height generator and then terrain composition based on that height alone, we should instead make a terrain shape and then composition based on that shape.
The shape generator will produce a chunk-sized array of bytes, distinguishing air, solid terrain and liquid terrain. Then, the composition will change the solid terrain into stone, dirt, sand, clay etc.

This will require changes in the structure generators as well, because some of those take the terrain height generator as input; these will need rewriting to take the terrain shape generator as input instead.