There's still some work to be done, but this contains the majority of the changes.

It's also likely to be a bit slower in some cases, but these should be easy to fix up.

* Introduce a webrender_traits crate to reduce build times (only servo and compositor depend on the webrender crate now).
* Support StackingContexts within Display Lists.
* Proper paint order for iframes.
* Much simpler (and more efficient) parallel batch creation.
* Split code into multiple source files - naming of various structs and files still needs to be tidied up.
* Change AABB tree to store items by draw list.
* Batch creation now builds a maximum of 1 VBO/VAO per AABB tree node.
* Moved internal display items (composite, clear) out of the public interface to an internal enum.
* Split into high level Scene struct and mid-level Frame representation.
* Switched draw lists to be managed by a free list, to reduce hash lookups.
* Support clipping for text elements.
* AABB tree now stores items that cross boundaries in multiple leaf nodes, and never in internal nodes.