For pass-through groups, we want to use the group's layer-stack
graph directly in its filter node, in place of the drawable's
buffer-source node.  Add a get_source_node() vfunc to GimpDrawable,
which defaults to returning the buffer-source node, and use it in
gimp_drawable_get_source_node() instead of using the buffer-source
node directly.  We'll later override this function for
GimpGroupLayer.