LibWeb: Let Paintable perform the painting

This patch adds a bunch of Paintable subclasses, each corresponding to
the Layout::Node subclasses that had a paint() override. All painting
logic is moved from layout nodes into their corresponding paintables.

Paintables are now created by asking a Layout::Box to produce one:

    static NonnullOwnPtr<Paintable> Layout::Box::create_paintable()

Note that inline nodes still have their painting logic. Since they
are not boxes, and all paintables have a corresponding box, we'll need
to come up with some other solution for them.

Userland/Libraries/LibWeb/Painting/ImagePaintable.h

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/Layout/ImageBox.h>
+#include <LibWeb/Painting/Paintable.h>
+
+namespace Web::Painting {
+
+class ImagePaintable final : public Paintable {
+public:
+    static NonnullOwnPtr<ImagePaintable> create(Layout::ImageBox const&);
+
+    virtual void paint(PaintContext&, PaintPhase) const override;
+
+    Layout::ImageBox const& layout_box() const;
+
+private:
+    ImagePaintable(Layout::ImageBox const&);
+};
+
+}