Fix bug where large rectangular tile appears

[jeffbaumes]

[manthey]

This is actually an error in the superpixel generation -- sometimes the code to try to avoid tile edge effects seems to miss labelling one or a few pixels and they get grouped into the "first" superpixel of that tile. With the refactor to parallelize superpixel generation, this will either be resolved or become a new incarnation of the issue. Until that refactor is in place, I recommend ignoring this. Once in place, if we see the issue again we can work on it then and if not, we can close this.