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
