In GimpImageProxy, implement the GimpPickable interface, so that
the proxy can be used as both a viewable and a pickable for the
image projection, with direct control over the show-all mode. This
will allow us to use a GimpImageProxy as input for a GimpLineArt.
(cherry picked from commit 0e02795128)
We're going to have GimpImageProxy implement GimpPickable, so that
it can be used as either a viewable or a pickable proxy for an
image.
(cherry picked from commit 8d8cc12f40)