LibWeb: Handle background-painting of Box in seperate function

Author: https://github.com/TobyAsE
Commit: d13526e1e7
Pull-request: https://github.com/SerenityOS/serenity/pull/7115
Reviewed-by: https://github.com/awesomekling

Userland/Libraries/LibWeb/Layout/Box.h

@@ -111,6 +111,7 @@ public:
 
     virtual void paint(PaintContext&, PaintPhase) override;
     virtual void paint_border(PaintContext& context);
+    virtual void paint_background(PaintContext& context);
 
     Vector<LineBox>& line_boxes() { return m_line_boxes; }
     const Vector<LineBox>& line_boxes() const { return m_line_boxes; }