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>