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/font.rs

@@ -69,7 +69,7 @@ pub trait FontTableMethods {
     fn with_buffer<F>(&self, F) where F: FnOnce(*const u8, usize);
 }
 
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Deserialize, Serialize)]
 pub struct FontMetrics {
     pub underline_size:   Au,
     pub underline_offset: Au,