Track rerender msgs

If the compositor outputs to a file:
* Track rerender msgs sent from the compositor to the render tasks.
* Before outputting, wait until all rerender msgs are processed by the
  render tasks.

Fixes issue https://github.com/servo/servo/issues/2871.

src/components/gfx/render_task.rs

@@ -242,6 +242,7 @@ impl<C:RenderListener + Send> RenderTask<C> {
                         debug!("render_task: render ready msg");
                         let ConstellationChan(ref mut c) = self.constellation_chan;
                         c.send(RendererReadyMsg(self.id));
+                        self.compositor.rerendermsg_discarded();
                         continue;
                     }