LibWeb: Move ImageDecoder client connection singleton to its own file

This will allow us to use it in more places around LibWeb.

Userland/Libraries/LibWeb/ImageDecoding.cpp

@@ -0,0 +1,23 @@
+/*
+ * Copyright (c) 2021, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/ImageDecoding.h>
+
+namespace Web {
+
+ImageDecoderClient::Client& image_decoder_client()
+{
+    static RefPtr<ImageDecoderClient::Client> image_decoder_client;
+    if (!image_decoder_client) {
+        image_decoder_client = ImageDecoderClient::Client::construct();
+        image_decoder_client->on_death = [&] {
+            image_decoder_client = nullptr;
+        };
+    }
+    return *image_decoder_client;
+}
+
+}