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/util/range.rs

@@ -124,7 +124,7 @@ macro_rules! int_range_index {
 }
 
 /// A range of indices
-#[derive(Clone, RustcEncodable, Copy)]
+#[derive(Clone, RustcEncodable, Copy, Deserialize, Serialize)]
 pub struct Range<I> {
     begin: I,
     length: I,