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.cpp

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2022, Dex♪ <dexes.ttp@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibImageDecoderClient/Client.h>
+#include <LibWebView/ImageDecoderClientAdapter.h>
+
+namespace WebView {
+
+NonnullRefPtr<ImageDecoderClientAdapter> ImageDecoderClientAdapter::create()
+{
+    return adopt_ref(*new ImageDecoderClientAdapter());
+}
+
+Optional<Web::ImageDecoding::DecodedImage> ImageDecoderClientAdapter::decode_image(ReadonlyBytes bytes)
+{
+    if (!m_client) {
+        m_client = ImageDecoderClient::Client::try_create().release_value_but_fixme_should_propagate_errors();
+        m_client->on_death = [&] {
+            m_client = nullptr;
+        };
+    }
+
+    auto result_or_empty = m_client->decode_image(bytes);
+    if (!result_or_empty.has_value())
+        return {};
+    auto result = result_or_empty.release_value();
+
+    Web::ImageDecoding::DecodedImage decoded_image;
+    decoded_image.is_animated = result.is_animated;
+    decoded_image.loop_count = result.loop_count;
+    for (auto const& frame : result.frames) {
+        decoded_image.frames.empend(move(frame.bitmap), frame.duration);
+    }
+
+    return decoded_image;
+}
+
+}