Biomes/decorations/ores: Make relative to 'water_level' setting

Biomes/decorations/ores: Make relative to 'water_level' setting

Add 'biome_zero_level' argument to 'generateBiomes()', 'deco_zero_level'
argument to 'placeAllDecos()' and 'ore_zero_level' to 'placeAllOres()'
to allow mapgens to vertically shift the registered biomes, decorations
and ores per-mapchunk.
Will also allow many realm possibilities in future mapgens.