Add double padding to fix missing faces on borders. Closes GH-13

Now matches the behavior of the terrain demo from voxel-mipmap-demo [1].
The voxel array is padded +4 air, two on each edge, one for the AO
calculations (voxel not rendered), and another (considered part of the
mesh) for rendering the adjacent voxel faces on the edges.

[1] https://github.com/mikolalysenko/voxel-mipmap-demo/blob/master/lib/examples.js

Requires deathcap/voxel-engine@216be651f223ff896067239e2907dbb0361112c9
and voxel-mesher ^0.11.0