Allow random_spheres to offset spheres from edge of image

[jgostick]

At the moment the options are fully contained spheres or spheres that extend beyond the image edge. It would be handy if we could also offset spheres from the image edges. This could either be done with a negative protrusion, or by adding a new edges mode, like 'padded', which would pad the edges with an amount equal to the clearance, for instance.