LibWeb: Share decoded images at the Resource level :^)

This patch adds ImageResource as a subclass of Resource. This new class
also keeps a Gfx::ImageDecoder so that we can share decoded bitmaps
between all clients of an image resource inside LibWeb.

With this, we now share both encoded and decoded data for images. :^)

I had to change how the purgeable-volatile flag is updated to keep the
volatile-images-outside-the-visible-viewport optimization working.
HTMLImageElement now inherits from ImageResourceClient (a subclass of
ResourceClient with additional image-specific stuff) and informs its
ImageResource about whether it's inside the viewport or outside.

This is pretty awesome! :^)

Libraries/LibWeb/Loader/ImageResource.cpp

@@ -0,0 +1,80 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <AK/Function.h>
+#include <LibGfx/Bitmap.h>
+#include <LibGfx/ImageDecoder.h>
+#include <LibWeb/Loader/ImageResource.h>
+
+namespace Web {
+
+ImageResource::ImageResource(const LoadRequest& request)
+    : Resource(Type::Image, request)
+{
+}
+
+ImageResource::~ImageResource()
+{
+}
+
+Gfx::ImageDecoder& ImageResource::ensure_decoder()
+{
+    if (!m_decoder)
+        m_decoder = Gfx::ImageDecoder::create(encoded_data());
+    return *m_decoder;
+}
+
+void ImageResource::update_volatility()
+{
+    if (!m_decoder)
+        return;
+
+    bool visible_in_viewport = false;
+    for_each_client([&](auto& client) {
+        if (static_cast<const ImageResourceClient&>(client).is_visible_in_viewport())
+            visible_in_viewport = true;
+    });
+
+    if (!visible_in_viewport) {
+        m_decoder->set_volatile();
+        return;
+    }
+
+    bool still_has_decoded_image = m_decoder->set_nonvolatile();
+    if (still_has_decoded_image)
+        return;
+
+    m_decoder = nullptr;
+    for_each_client([&](auto& client) {
+        static_cast<ImageResourceClient&>(client).resource_did_replace_decoder();
+    });
+}
+
+ImageResourceClient::~ImageResourceClient()
+{
+}
+
+}