LibWeb: Respect CSS z-index property while painting

To support z-ordering when painting, the layout tree now has a parallel
sparse tree of stacking contexts. The rules for which layout boxes
establish a stacking context are a bit complex, but the intent is to
encapsulate the decision making into establishes_stacking_context().

When we paint, we start from the ICB (LayoutDocument) who always has a
StackingContext and then paint the tree of StackingContexts where each
node has its children sorted by z-index.

This is pretty crude, but gets the basic job done. Note that this does
not yet support hit testing; hit testing is still done using a naive
treewalk from the root.

Libraries/LibWeb/Layout/LayoutDocument.h

@@ -40,6 +40,8 @@ public:
     virtual const char* class_name() const override { return "LayoutDocument"; }
     virtual void layout(LayoutMode = LayoutMode::Default) override;
 
+    virtual void render(RenderingContext&) override;
+
     const LayoutRange& selection() const { return m_selection; }
     LayoutRange& selection() { return m_selection; }
 
@@ -47,8 +49,17 @@ public:
 
     virtual bool is_root() const override { return true; }
 
+    void build_stacking_context_tree();
+
 private:
     LayoutRange m_selection;
 };
 
+template<>
+inline bool is<LayoutDocument>(const LayoutNode& node)
+{
+    return node.is_root();
+}
+
+
 }