script: Do not set up an IPC route for every image load (#35041)

Instead of setting up a route for every image load in the DOM / Layout,
route all incoming image cache responses through the `ScriptThread`.
This avoids creating a set of file descriptor for every image that is
loaded.

This change requires having the `ImageCache` track the `PipelineId` of
the original the listener so that the `ScriptThread` can route it
properly to the correct `Window`.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/layout/context.rs

@@ -137,7 +137,7 @@ impl LayoutContext<'_> {
                     Some(image_info)
                 }
             },
-            None | Some(ImageOrMetadataAvailable::MetadataAvailable(_)) => None,
+            None | Some(ImageOrMetadataAvailable::MetadataAvailable(..)) => None,
         }
     }
 }