In GimpProjectable, replace gimp_projectable_get_size(), which only
returned a width and a height, with
gimp_projectable_get_bounding_box(), which returns a full
rectangle.  This allows projectables to have an arbitrary bounding
box, not limited to a (0, 0) top-left corner.

Adapt GimpProjection, creating a buffer with corresponding extent
to the projectable's bounding box.

Adapt GimpImage and GimpGroupLayer.