Deadlock when changing surface of surface selector in framegraph.

This happened after the OnDemand fixes and addition of barriers.

When setting the framegraph surface to nullptr and then resetting it to a valid surface, nothing happens. The render thread is hanging in Renderer::waitForRenderJobs(), so I assume it is a deadlock. 

This is easy to reproduce. I modified the basicshapes-cpp example by adding two buttons, one that sets the surface in the framegraph surface selector to null, the other to the original surface.