Fill Holes in a Mask

Fill in holes that are surrounded by the mask. Any white space (outside the mask) that is accessible from the boundary is kept, but any white space that is not reachable from the boundary is added to the mask.

What that is useful for is that if you have a cell where you get the boundary, but there are internal areas that are not part of the mask, this can fill them in without rounding sharp corners at all.