In GimpSourceCore, when applying a symmetry transform to the source
content, combine the transform op with translation to the paint-
buffer coordinates, so that subclasses (namely, GimpClone and
GimpHeal) can use the op to apply the transformation directly from
the source buffer to the paint buffer in a single step.  This is
both more efficient, and avoids incorrect symmetry transforms when
the paint buffer is cropped to the bounds of the drawable.