The function which calculates positions of an empty square currently has a bug:
it returns the northeastern and southwestern corners twice
and omits the northwestern and southeastern ones.
This leads to a nether portal detection which is not rigorous enough, skipping eight nodes.
This commit fixes the problem.
Closes #13
Mapgen: Find a way to get the perlin noise values inside [-1; 1] or use
another noise.
The problem is visible in the nether forest, where the mapgen code flattens
the ceiling if it is very high.
Mapgen: Generate more detail inside the simple pyramid-like "buildings",
e.g. add a small treasure chest node which contains items