gfx: Make display lists serializable using serde.

This commit introduces the `serde` dependency, which we will use to
serialize messages going between processes in multiprocess Servo.

This also adds a new debugging flag, `-Z print-display-list-json`,
allowing the output of display list serialization to be visualized.
This will be useful for our experiments with alternate rasterizers.

components/gfx/lib.rs

@@ -14,9 +14,11 @@
 #![feature(vec_push_all)]
 
 #![plugin(plugins)]
+#![plugin(serde_macros)]
 
 #[macro_use]
 extern crate log;
+extern crate serde;
 
 extern crate azure;
 #[macro_use] extern crate bitflags;