Why do we need this?

If core layout runs before grid routing, then grid routing can just see that routes are already created and not make the straight connections.

So the issue is that core layout doesn't run with nested graphs otherwise it would misplace nested sequence diagrams or grid cells when running its own layout. After we run layout on the diagram without nested graphs, we then re-inject them and route the cross-graph edges including edges to descendants that do not exist while running core layout.

With this change, we can now pass the plugin the graph with descendants and have it only perform the edge routing for the specified edges. This way the plugin can implement its own edge routing with all the nested graphs present and not be affecting the positioning of any pre-existing shapes and edges.