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/script/lib.rs

@@ -37,7 +37,6 @@ pub(crate) mod document_loader;
 mod dom;
 mod canvas_state;
 pub(crate) mod fetch;
-mod image_listener;
 mod init;
 mod layout_image;