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
This ensures that the layout viewport responds to each type of zoom correctly,
and lays the ground for CSS Media Queries and CSS Device Adaption.
Until we have proper touch support, mobile-style "pinch" zoom can be simulated
by holding Ctrl while scrolling with a mousewheel or trackpad gesture.
