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/CSS/StyleValue.cpp

@@ -293,7 +293,7 @@ ImageStyleValue::ImageStyleValue(const URL& url, Document& document)
 {
     LoadRequest request;
     request.set_url(url);
-    set_resource(ResourceLoader::the().load_resource(request));
+    set_resource(ResourceLoader::the().load_resource(Resource::Type::Generic, request));
 }
 
 void ImageStyleValue::resource_did_load()