In GimpGroupLayer, when recalculating the group's size as a result
of a change to one of the child layers (now including in response
to a child layer's GimpDrawable::bounding-box-changed signal),
calculate the group's bounding box (the bounding box of all its
child layers' bounding boxes) alongside its logical bounds.  Like
in GimpLayer, use the logical bounds as the bounding box if the
group has a mask.

This bounding box is passed to the group's projection, via
GimpGroupLayer's GimpProjectable::get_bounding_box()
implementation, resulting in a buffer whose extent is the same as
the bounding box.