First pass of refactoring, batch improvements and api updates.

[glennw]

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.