Support WebDriver takeScreenshot command.

This adds support for compositing to a PNG without actually quiting
the browser.

components/msg/Cargo.toml

@@ -32,6 +32,9 @@ git = "https://github.com/servo/rust-core-foundation"
 [dependencies.io_surface]
 git = "https://github.com/servo/rust-io-surface"
 
+[dependencies.png]
+git = "https://github.com/servo/rust-png"
+
 [dependencies]
 url = "0.2.16"
 bitflags = "*"

components/msg/constellation_msg.rs

@@ -11,6 +11,7 @@ use geom::scale_factor::ScaleFactor;
 use hyper::header::Headers;
 use hyper::method::Method;
 use layers::geometry::DevicePixel;
+use png;
 use util::cursor::Cursor;
 use util::geometry::{PagePx, ViewportPx};
 use std::sync::mpsc::{channel, Sender, Receiver};
@@ -233,10 +234,12 @@ pub enum Msg {
     Focus(PipelineId),
     /// Requests that the constellation retrieve the current contents of the clipboard
     GetClipboardContents(Sender<String>),
-    // Dispatch a webdriver command
+    /// Dispatch a webdriver command
     WebDriverCommand(PipelineId, WebDriverScriptCommand),
     /// Notifies the constellation that the viewport has been constrained in some manner
     ViewportConstrained(PipelineId, ViewportConstraints),
+    /// Create a PNG of the window contents
+    CompositePng(Sender<Option<png::Image>>)
 }
 
 #[derive(Clone, Eq, PartialEq)]

components/msg/lib.rs

@@ -7,6 +7,7 @@ extern crate azure;
 extern crate geom;
 extern crate hyper;
 extern crate layers;
+extern crate png;
 extern crate util;
 extern crate url;
 extern crate style;