Auto merge of #345 - glennw:wr2-occlusion-fix, r=pcwalton
Auto merge of #345 - glennw:wr2-occlusion-fix, r=pcwalton

Ensure items outside world clip rect of layers aren't added to tiles.

This is both an optimization and a bug fix :)

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/webrender/345)
<!-- Reviewable:end -->

servo/webrender,servo/webrender,alanamramjit/webrender,servo/webrender,servo/webrender,alanamramjit/webrender,servo/webrender,servo/webrender