Impelemented zooming and slightly better scrolling.
Merge with pcwalton's updated branch
Changed texturing mode to LINEAR for better zooming
Merge onto mozilla/servo master
Merge branch 'master' of https://github.com/mozilla/servo
Merge branch 'master' of https://github.com/mozilla/servo
Add quadtree implementation
auto merge of #529 : eschweic/servo/master, r=pcwalton

Add a quadtree implementation to allow the compositor to keep track of which tiles have been rendered and which haven't.

r? @pcwalton