LibCore: Make EventReceiver ref counting use atomics

This removes a race in ImageDecoder's make_decode_image_job(), where the
ref count of `this` is mutated from separate threads in the callbacks.

Libraries/LibCore/EventReceiver.h

@@ -1,12 +1,13 @@
 /*
- * Copyright (c) 2018-2021, Andreas Kling <[email protected]>
+ * Copyright (c) 2018-2024, Andreas Kling <[email protected]>
  * Copyright (c) 2022, the SerenityOS developers.
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #pragma once
 
+#include <AK/AtomicRefCounted.h>
 #include <AK/ByteString.h>
 #include <AK/Forward.h>
 #include <AK/Function.h>
@@ -50,7 +51,7 @@ public:                                            \
     }
 
 class EventReceiver
-    : public RefCounted<EventReceiver>
+    : public AtomicRefCounted<EventReceiver>
     , public Weakable<EventReceiver> {
     // NOTE: No C_OBJECT macro for Core::EventReceiver itself.