Switch to using WebRender hit testing

This trades quite a bit of complicated code in Servo for few more
messages and a significant performance improvement. In particular,
WebRender can search the entire display list at once instead of
ping-ponging down the pipeline tree. This allows us to send mouse
events to the correct pipeline immediately.

components/compositing/lib.rs

@@ -9,6 +9,7 @@ extern crate gfx_traits;
 extern crate gleam;
 extern crate image;
 extern crate ipc_channel;
+extern crate libc;
 #[macro_use]
 extern crate log;
 extern crate msg;