ogt_voxel_meshify: Suppress geometry between chunks

[jpaver]

For meshing minecraft style chunks, the current algorithms will always generate polygons on chunk boundaries as they assume the data outside the grid is non-solid.

It may be useful to add support for avoiding these polygons.

One possible way would be if user tries to meshify MxNxK voxels, but provides M+2xN+2xK+2 voxel grid that contain the neighbor cells within the outer-shell. It'd be simple enough to extend the meshifiers to only consider the inner MxNxK grid for meshing, but use the data on the shell for determining whether faces are required.