voxel2mesh fixes

[Lestropie]

Two minor glitches I found in voxel2mesh when diagnosing issues reported by a colleague.

[Lestropie]

For discoverability: what is rectified in f666c76 is that feeding the result to meshfilter would result in "Cannot perform smoothing on this mesh: no triangulation information" from here. The original voxel2mesh -blocky implementation just generated vertices at voxel corners as required, resulting in duplicate vertices; a later change was supposed to re-use existing vertices so that the resulting surface would be closed, but was not functional.