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/msg/compositor_msg.rs

@@ -60,7 +60,7 @@ pub enum LayerKind {
 }
 
 /// The scrolling policy of a layer.
-#[derive(Clone, PartialEq, Eq, Copy)]
+#[derive(Clone, PartialEq, Eq, Copy, Deserialize, Serialize)]
 pub enum ScrollPolicy {
     /// These layers scroll when the parent receives a scrolling message.
     Scrollable,