LibWeb: Make Document::paintable() return a ViewportPaintable

Author: https://github.com/awesomekling
Commit: 25375bf1d5
Pull-request: https://github.com/SerenityOS/serenity/pull/20661

Userland/Libraries/LibWeb/DOM/Node.h

@@ -185,6 +185,7 @@ public:
     Painting::PaintableBox const* paintable_box() const;
     Painting::PaintableBox* paintable_box();
     Painting::Paintable const* paintable() const;
+    Painting::Paintable* paintable();
 
     void set_layout_node(Badge<Layout::Node>, JS::NonnullGCPtr<Layout::Node>);
     void detach_layout_node(Badge<Layout::TreeBuilder>);