layout: Make the LocalImageCache on_image_available callback not use

`@`.

This is one small step toward parallel layout.

src/components/main/layout/layout_task.rs

@@ -675,15 +675,15 @@ impl LayoutTask {
     // to the script task, and ultimately cause the image to be
     // re-requested. We probably don't need to go all the way back to
     // the script task for this.
-    fn make_on_image_available_cb(&self) -> @ImageResponder {
+    fn make_on_image_available_cb(&self) -> ~ImageResponder:Send {
         // This has a crazy signature because the image cache needs to
         // make multiple copies of the callback, and the dom event
         // channel is not a copyable type, so this is actually a
         // little factory to produce callbacks
-        @LayoutImageResponder {
+        ~LayoutImageResponder {
             id: self.id.clone(),
             script_chan: self.script_chan.clone(),
-        } as @ImageResponder
+        } as ~ImageResponder:Send
     }
 
     /// Handles a message to destroy layout data. Layout data must be destroyed on *this* task

src/components/net/local_image_cache.rs

@@ -33,7 +33,7 @@ pub fn LocalImageCache(image_cache_task: ImageCacheTask) -> LocalImageCache {
 pub struct LocalImageCache {
     priv image_cache_task: ImageCacheTask,
     priv round_number: uint,
-    priv on_image_available: Option<@ImageResponder>,
+    priv on_image_available: Option<~ImageResponder:Send>,
     priv state_map: UrlMap<@mut ImageState>
 }
 
@@ -47,7 +47,7 @@ struct ImageState {
 impl LocalImageCache {
     /// The local cache will only do a single remote request for a given
     /// URL in each 'round'. Layout should call this each time it begins
-    pub fn next_round(&mut self, on_image_available: @ImageResponder) {
+    pub fn next_round(&mut self, on_image_available: ~ImageResponder:Send) {
         self.round_number += 1;
         self.on_image_available = Some(on_image_available);
     }
@@ -113,7 +113,7 @@ impl LocalImageCache {
                 // on the image to load and triggering layout
                 let image_cache_task = self.image_cache_task.clone();
                 assert!(self.on_image_available.is_some());
-                let on_image_available = self.on_image_available.unwrap().respond();
+                let on_image_available = self.on_image_available.as_ref().unwrap().respond();
                 let url = (*url).clone();
                 do task::spawn {
                     let (response_port, response_chan) = comm::stream();