Canvas layer clipping

This PR add a precompose event and a layer clipping example.

![layer-clipping](https://f.cloud.github.com/assets/76594/1529636/ba439704-4c3a-11e3-9b44-e7d21d969d19.png)

Tom, please merge (or cherry-pick) if you think this is a appropriate for the canvas-exp branch.