Biome generation: Allow mapgens to alter biome zero level

[paramat]

WIP still need to test.

Allows mapgens to raise or lower the entire set of registered biomes per-mapchunk by setting the 'biome zero level' instead of that level always being y = 0.
This will allow the option of repeating the registered set of biomes in mgv7 floatlands without needing to register a duplicate set of biomes just for floatlands.
Will also allow future mapgens to create a world of multiple stacked realms without needing to register a set of biomes for every realm.
Will probably allow further interesting possibilites for future mapgens.
(I am currently experimenting with new core mapgens that can make use of this PR).

Also intended in other PRs: The same feature for registered ores and decorations.

[paramat]

Works for mgv7 floatlands with MTGame floatland biomes disabled and:

 	// Init biome generator, place biome-specific nodes, and build biomemap
 	biomegen->calcBiomeNoise(node_min);
-	MgStoneType stone_type = generateBiomes(0);
+	s16 biome_zero_y = ((spflags & MGV7_FLOATLANDS) &&
+			node_min.Y > shadow_limit) ?
+			floatland_level - 1 : 0;
+	MgStoneType stone_type = generateBiomes(biome_zero_y);

[paramat]

Build failed for silly reasons.

[Fixer-007]

Will also allow future mapgens to create a world of multiple stacked realms without needing to register a set of biomes for every realm.

Terrain and biomes will be random on both realm?

[paramat]

Hmm unfortunately if biomes repeat they will have identical distribution on each level because heat and humidity noises are 2D noises. So now i'm unsure about this PR, we are intending to have differing biomes for mgv7 floatlands.

Terrain is independent from biomes and is created by a core mapgen, my current experimental core mapgen has the same character of terrain on each level but the terrain is not identical on each level. The noise used is shifted by 62000 nodes per level so no terrain on a level will match any other terrain on any other level, every piece of terrain on every level is completely unique.

Ii might be possible to shift heat and humidity noises in the same way.
EDIT: Ninja'd.

[Fixer-007]

Why not shift (a lot) biomes too?

[paramat]

Yeah, just edited my comment above with that.

[paramat]

What this comes down to is, is it interesting to have the same set of biomes used on every level? I would prefer differing biomes per level.
For now i'll close this and instead work on a more interesting biome feature (which would conflict anyway).

[paramat]

Will resurrect this in a new PR as is very useful.