Pad masks before growing halos, or trim-size edges flatten

If a die-cut preview grows a white edge with PIL MaxFilter or a similar filter inside the image's own bounds, any shape touching the file edge gets clipped flat on that side. Artwork with bleed hides this, but trim-size artwork with no bleed shows straightened edges on circles and curves. Expand the mask canvas by the halo radius before filtering. The defect looks like damaged artwork, so check preview code before suspecting the source files.