layout/context: Wrap image_cache_thread in a Mutex<> as well

`SharedLayoutContext.image_cache_thread` didn't get the `Mutex<>`
treatment along with all the other channels in there, because
`ipc-channel::Sender` is presently inherently `Sync`. I believe this to
be an implementation accident though, that should be rectified in the
future -- not something users should actually rely on...

Note that sharing senders (be it `mpsc` or `ipc-channel`) in is probably
not a good idea anyway: just cloning them -- and letting them handle the
sharing internally -- should be both simpler and cheaper. But right now
that's how things are handled here; so let's go with the flow...

components/layout_thread/lib.rs

@@ -513,7 +513,7 @@ impl LayoutThread {
                 local_context_creation_data: Mutex::new(local_style_context_creation_data),
                 timer: self.timer.clone(),
             },
-            image_cache_thread: self.image_cache_thread.clone(),
+            image_cache_thread: Mutex::new(self.image_cache_thread.clone()),
             image_cache_sender: Mutex::new(self.image_cache_sender.clone()),
             font_cache_thread: Mutex::new(self.font_cache_thread.clone()),
             webrender_image_cache: self.webrender_image_cache.clone(),