Add resize functionality to quadtrees
Allow rectangular tiles
Add compositor_layer.rs
Refactor compositor; implement compositor iframe support
Update submodule pointers; address nits; add optional page_size
auto merge of #675 : eschweic/servo/multi-layer, r=metajack

This refactors the compositor and adds initial support for multiple compositor layers being displayed at once. A new module, `compositor_layer.rs`, does most of the heavy lifting that was once in `mod.rs`.

--- clipped so bors is happy ---