Remove center vertex, speeding up meshing by ~15%.

The node profiler tools didn't seem to think this function took very much
time, so I didn't even touch it before, but it seems to have much more
of an effect than any of the other functions the profiler thought were
a big deal. Makes the profiler kind of useless, but I digress.
Update benchmarks to have chunk neighbours.
Better mesh benchmarking, we can now use chrome developer tools to profile.
10x meshing speedup.

Used lookup tables rather than code, + moved lookup table definition outside function.
Add back center vertex, effect on speed is actually neglegable.
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Merge pull request #166 from crazy2be/faster-meshing-2

Faster meshing 2