Support extra space for convolution in facets

This MR adds a third type of boundary boxes around a facet. In addition to the trimmed and untrimmed boxes, a facet now also has a convolution box, which adds a fixed amount of space around the trimmed box. Instead of the feathering space, this fixed space is not limited to the size of the image, because (unlike feathering) also the borders of the full image require this space to prevent wrapping during the convolution done for feathering.