Clip function now respects pixel ratio.

When clipping on a screen with a pixel ratio other than 1 users were experiencing a bogus offset compared to screens with a pixel ratio of 1.

![Google Chrome](https://f.cloud.github.com/assets/437794/872306/4b08d0c6-f860-11e2-9202-bf6a4be3285d.png)
![Safari (iPad 4)](https://f.cloud.github.com/assets/437794/872307/545ac4cc-f860-11e2-8492-d2782ef17bd5.PNG)