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/cursor.rs

@@ -9,7 +9,7 @@ use std::ascii::AsciiExt;
 
 macro_rules! define_cursor {
     ($( $css: expr => $variant: ident = $value: expr, )+) => {
-        #[derive(Clone, Copy, PartialEq, Eq, Debug)]
+        #[derive(Clone, Copy, PartialEq, Eq, Debug, Deserialize, Serialize)]
         #[repr(u8)]
         pub enum Cursor {
             $( $variant = $value ),+