Remove extraneous references to flow tree objects.
Switch flow tree to owned pointers
Fix nits; warning police
auto merge of #721 : eric93/servo/parallel-layout-ownedtree, r=metajack

Don't store flows and boxes in DOM nodes and display lists.
Stop copying buffers when sending them to quadtrees
Add buffer_map.rs; implement LayerBuffer recycling
Change quadtree iterators