Add gimp_drawable_filter_set_crop(), which allows setting an output
crop rectangle for the filter; anything outside the rectangle
doesn't get filtered.  The crop area is combined with the preview
area to determine the filtered area during preview, however, unlike
the preview area, the crop area remains in effect while committing
the filter.

Consequently, when merging a drawable filter, if the filter has a
crop, only process the cropped area.