Restructure compositor layers to work with iframes

When a frame is selected via set_ids, a tree of root compositor
layers is also created, matching the tree of pipelines in the frame.
This decouples the chronological ordering dependency for parent frames
and child iframes sending CreateOrUpdateRootLayer &
CreateOrUpdateDescendentLayer messages.

Change the Compositor ready and render states to per pipeline.
This ensures the compositor doesn't composite for an epoch until
every *pipeline* in the epoch is finished rendering.

For iframes it fixes a bug where the compositor didnt wait on the
child pipeline ready state before compositing the window.

Gotchas:
* layout task or script task failure on exit ("task '' failed at sending
  on a closed channel"), this happens if the child iframe shares the
  same script task as the parent and can be avoided by adding the
  sandbox attribute to the iframe.

Other changes:
* Inline set_clipping_rect in compositor.rs
* Commented out ref test simple_iframe.html fails on os x

src/components/script/page.rs

@@ -319,7 +319,7 @@ impl Page {
                 self.join_layout();
 
                 // Tell the user that we're performing layout.
-                compositor.set_ready_state(PerformingLayout);
+                compositor.set_ready_state(self.id, PerformingLayout);
 
                 // Layout will let us know when it's done.
                 let (join_chan, join_port) = channel();