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/net_traits/Cargo.toml

@@ -24,4 +24,6 @@ log = "*"
 url = "0.2.35"
 hyper = "0.6"
 euclid = "0.1"
+serde = "*"
+serde_macros = "*"
 

components/net_traits/image/base.rs

@@ -10,6 +10,7 @@ use util::vec::byte_swap;
 // FIXME: Images must not be copied every frame. Instead we should atomically
 // reference count them.
 
+#[derive(Deserialize, Serialize)]
 pub enum PixelFormat {
     K8,         // Luminance channel only
     KA8,        // Luminance + alpha
@@ -17,6 +18,7 @@ pub enum PixelFormat {
     RGBA8,      // RGB + alpha, 8 bits per channel
 }
 
+#[derive(Deserialize, Serialize)]
 pub struct Image {
     pub width: u32,
     pub height: u32,

components/net_traits/lib.rs

@@ -3,15 +3,19 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #![feature(box_syntax)]
+#![feature(custom_derive)]
+#![feature(plugin)]
 #![feature(slice_patterns)]
 #![feature(step_by)]
 #![feature(vec_push_all)]
+#![plugin(serde_macros)]
 
 extern crate euclid;
 extern crate hyper;
 #[macro_use]
 extern crate log;
 extern crate png;
+extern crate serde;
 extern crate stb_image;
 extern crate url;
 extern crate util;