Delete WR images when dropping the ImageCacheStore (#36956)

This deletes images from WR when dropping the ImageCacheStore for a
WebView.

Testing: Run `./mach run --enable-experimental-web-platform-features
unsplash.com` and then open `about:memory` in a new tab. On Linux we end
up with ~30MB of WR images. Then close the unsplash.com tab and measure
memory again, it will down to ~1.25MB

Fixes: https://github.com/servo/servo/issues/25927

Signed-off-by: webbeef <me@webbeef.org>

components/net/image_cache.rs

@@ -7,7 +7,7 @@ use std::collections::hash_map::Entry::{Occupied, Vacant};
 use std::sync::{Arc, Mutex};
 use std::{mem, thread};
 
-use compositing_traits::{CrossProcessCompositorApi, SerializableImageData};
+use compositing_traits::{CrossProcessCompositorApi, ImageUpdate, SerializableImageData};
 use imsz::imsz_from_reader;
 use ipc_channel::ipc::IpcSharedMemory;
 use log::{debug, warn};
@@ -675,6 +675,20 @@ impl ImageCache for ImageCacheImpl {
     }
 }
 
+impl Drop for ImageCacheStore {
+    fn drop(&mut self) {
+        let image_updates = self
+            .completed_loads
+            .values()
+            .filter_map(|load| match &load.image_response {
+                ImageResponse::Loaded(image, _) => image.id.map(ImageUpdate::DeleteImage),
+                _ => None,
+            })
+            .collect();
+        self.compositor_api.update_images(image_updates);
+    }
+}
+
 impl ImageCacheImpl {
     /// Require self.store.lock() before calling.
     fn add_listener_with_store(&self, store: &mut ImageCacheStore, listener: ImageResponder) {