Fix Quadtree tests

Ever since my earlier refactoring, the Quadtree tests have been broken.
Fix this by re-templatizing Quadtree, since the unit tests rely on this
functionality.

Fixes #2768.
Merge pull request #80 from mrobinson/fix-make-check

Fix Quadtree tests
Layers store their bounds

Instead of only storing their origin, layers should store their complete
boundaries, to avoid duplication between Servo and rust-layers.
Remove page_size argument for layer creation

Servo will no longer pass the page_size argument during layer creation,
since the page_size is always equal to the bounding rect.