reduced memory footprint of marching_cubes
reduced memory footprint of marching_cubes

`memory size_t *shared_indices` was the memory limiting var in
marching_cubes and took 24byte per voxel while rendering the mesh.
Since the var is only needed for i and (i - 1) while processing in x
dimension, the x dimension is removable, see usage of i_mod_2 and
i_mod_2_inv

pmneila/PyMCubes,pmneila/PyMCubes,pmneila/PyMCubes