Make quadtrees generic
Address pcwalton's comments
Implement progressive rendering
Clean up and annotations
Cleanup after merge
Fix paint permissions issue, clean up
Address metajack's comments
Move buffer requests to quadtree; add remove_tile() to quadtree; automatic rerendering after pinch-zoom
Address pcwalton's nits; automatic rendering after scrolling
auto merge of #557 : eschweic/servo/master, r=pcwalton

The compositor now keeps track of what has been rendered and what needs rendering based on the window rect, and asks the renderer for tiles only when needed.