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/CMakeLists.txt

@@ -83,6 +83,7 @@ set(SOURCES
     Layout/LayoutWidget.cpp
     Layout/LineBox.cpp
     Layout/LineBoxFragment.cpp
+    Loader/ImageResource.cpp
     Loader/Resource.cpp
     Loader/ResourceLoader.cpp
     PageView.cpp