Better display of the "void" volume between segments

Currently, any value not in a segment gets discarded. A potential alternative approach is to zoom/interpolate the volume corresponding to label 0, *and* divide it by `factor**ndim`, simulating diffusion of the image intensity as it gets stretched. This will be useful to spot missed segments as the volume expands, for example.

This approach will yield black "halos" around the objects of interest but this cannot be avoided without complex nonlinear warping, which I don't want to get into. :joy:

CC @haesleinhuepf — this is a case where Clesperanto will *certainly* be much faster than ndimage.