Mgv5 1 up 1 down overgeneration for biome surface continuity, thinner biomes.

![screenshot_3488555948](https://cloud.githubusercontent.com/assets/3686677/5136687/5e9db794-7124-11e4-9e70-babed4bce9da.png)


A new pull request for a simplified overgeneration, extending 1 up 1 down beyond the mapchunk. This makes biome surface continuous at y = 47, although the full depth of the biome is still interrupted.
The biome API needs some work to make it (more) continuous across mapchunk borders, in the meantime this commit fixes the ugly line of missing biome nodes at y = 47.

At line 401/402 i made a change to the depth of biomes, 'biome->depth_filler' was used twice in the depth calculation, perhaps on purpose, but it results in very deep biomes not averaging at the value set in the API.