LibWeb: Draw elements with opacity in a separate stacking context

Author: https://github.com/Cleverking2003
Commit: f567414f65
Pull-request: https://github.com/SerenityOS/serenity/pull/8956
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/alimpfard

Userland/Libraries/LibWeb/Painting/StackingContext.h

@@ -35,6 +35,8 @@ private:
     Box& m_box;
     StackingContext* const m_parent { nullptr };
     Vector<StackingContext*> m_children;
+
+    void paint_internal(PaintContext&);
 };
 
 }