LibWeb: Add ViewportPaintable to represent viewports in the paint tree

This patch just adds the new root paintable and updates the tests
expectations. The next patch will move painting logic from the layout
viewport to the paint viewport.

Userland/Libraries/LibWeb/Painting/ViewportPaintable.h

@@ -0,0 +1,24 @@
+/*
+ * Copyright (c) 2023, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/Painting/PaintableBox.h>
+
+namespace Web::Painting {
+
+class ViewportPaintable final : public PaintableWithLines {
+    JS_CELL(ViewportPaintable, PaintableWithLines);
+
+public:
+    static JS::NonnullGCPtr<ViewportPaintable> create(Layout::Viewport const&);
+    virtual ~ViewportPaintable() override;
+
+private:
+    explicit ViewportPaintable(Layout::Viewport const&);
+};
+
+}