LibWeb: Abstract the image decoding via Web::ImageDecoding::Decoder

After this change, LibWeb now expects Web::ImageDecoding::Decoder to be
pre-initialized with a concrete implementation before using the webpage.
The previous implementation, based on the ImageDecoder service, has been
provided directly through an adapter in LibWebClient, and is now used as
the default value by WebContent.

Userland/Libraries/LibWebView/ImageDecoderClientAdapter.h

@@ -0,0 +1,32 @@
+/*
+ * Copyright (c) 2022, Dex♪ <dexes.ttp@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/NonnullRefPtr.h>
+#include <LibWeb/ImageDecoding.h>
+
+namespace ImageDecoderClient {
+class Client;
+}
+
+namespace WebView {
+
+class ImageDecoderClientAdapter : public Web::ImageDecoding::Decoder {
+public:
+    static NonnullRefPtr<ImageDecoderClientAdapter> create();
+
+    virtual ~ImageDecoderClientAdapter() override = default;
+
+    virtual Optional<Web::ImageDecoding::DecodedImage> decode_image(ReadonlyBytes) override;
+
+private:
+    explicit ImageDecoderClientAdapter() = default;
+
+    RefPtr<ImageDecoderClient::Client> m_client;
+};
+
+}