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/platform/macos/font.rs

@@ -64,7 +64,7 @@ impl FontHandleMethods for FontHandle {
             Some(s) => s.to_f64_px(),
             None => 0.0
         };
-        match template.ctfont {
+        match *template.ctfont {
             Some(ref ctfont) => {
                 Ok(FontHandle {
                     font_data: template.clone(),