Generator shape refactor

The generators were rewritten so that a new pipeline is used, the cTerrainShapeGenerator takes the biomes and generates the overall terrain shape (air vs non-air blocks), then this shape is handed off to composition generators, which fill it with biome-specific blocks.
This way the overhangs may be generated by any shape generator and the terrain composed by any other compositor. In the past any overhang (3D) generator had to be a composition generator.

Together with this change the generator defaults were changed to `BiomalNoise3D` shape generator and `Biomal` composition generator.

Closes #1591.
Fixes #1613.