LibWeb: Add non-const version of paintable_box() in Layout::Node

Allows to remove a bunch of const_cast's

Userland/Libraries/LibWeb/Layout/Box.cpp

@@ -77,6 +77,11 @@ JS::GCPtr<Painting::Paintable> Box::create_paintable() const
     return Painting::PaintableBox::create(*this);
 }
 
+Painting::PaintableBox* Box::paintable_box()
+{
+    return static_cast<Painting::PaintableBox*>(Node::paintable());
+}
+
 Painting::PaintableBox const* Box::paintable_box() const
 {
     return static_cast<Painting::PaintableBox const*>(Node::paintable());

Userland/Libraries/LibWeb/Layout/Box.h

@@ -23,6 +23,7 @@ class Box : public NodeWithStyleAndBoxModelMetrics {
 
 public:
     Painting::PaintableBox const* paintable_box() const;
+    Painting::PaintableBox* paintable_box();
 
     virtual void set_needs_display() override;
 

Userland/Libraries/LibWeb/Layout/Viewport.cpp

@@ -33,24 +33,24 @@ void Viewport::build_stacking_context_tree_if_needed()
 
 void Viewport::build_stacking_context_tree()
 {
-    const_cast<Painting::PaintableBox*>(paintable_box())->set_stacking_context(make<Painting::StackingContext>(*this, nullptr, 0));
+    paintable_box()->set_stacking_context(make<Painting::StackingContext>(*this, nullptr, 0));
 
     size_t index_in_tree_order = 1;
     for_each_in_subtree_of_type<Box>([&](Box& box) {
         if (!box.paintable_box())
             return IterationDecision::Continue;
-        const_cast<Painting::PaintableBox*>(box.paintable_box())->invalidate_stacking_context();
+        box.paintable_box()->invalidate_stacking_context();
         if (!box.establishes_stacking_context()) {
             VERIFY(!box.paintable_box()->stacking_context());
             return IterationDecision::Continue;
         }
-        auto* parent_context = const_cast<Painting::PaintableBox*>(box.paintable_box())->enclosing_stacking_context();
+        auto* parent_context = box.paintable_box()->enclosing_stacking_context();
         VERIFY(parent_context);
-        const_cast<Painting::PaintableBox*>(box.paintable_box())->set_stacking_context(make<Painting::StackingContext>(box, parent_context, index_in_tree_order++));
+        box.paintable_box()->set_stacking_context(make<Painting::StackingContext>(box, parent_context, index_in_tree_order++));
         return IterationDecision::Continue;
     });
 
-    const_cast<Painting::PaintableBox*>(paintable_box())->stacking_context()->sort();
+    paintable_box()->stacking_context()->sort();
 }
 
 void Viewport::paint_all_phases(PaintContext& context)