Memory issue: WR image resources are not deleted

[paulrouget]

Even after the image cache has been destroyed, the WR image resources are still in memory.

Sending a delete_image transaction appears to solve the issue, but I'm not exactly sure when we should tell WR to drop the images.

The options are:

• when the pipeline is destroyed
• when ImageCacheStore is destroyed
• when the image fragment are destroyed
• … any other time?

--- a/components/net/image_cache.rs
+++ b/components/net/image_cache.rs
@@ -342,6 +342,22 @@ struct ImageCacheStore {
     webrender_api: WebrenderIpcSender,
 }

+impl Drop for ImageCacheStore {
+    fn drop(&mut self) {
+        for load in self.completed_loads.values() {
+            if let ImageResponse::Loaded(image, _) = &load.image_response {
+                if let Some(image_key) = image.id {
+                    let mut txn = webrender_api::Transaction::new();
+                    txn.delete_image(image_key);
+                    self.webrender_api.update_resources(txn.resource_updates);
+                }
+            }
+        }
+    }
+}