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/LayoutNode.cpp

@@ -94,8 +94,9 @@ void LayoutNode::render(RenderingContext& context)
     if (!is_visible())
         return;
 
-    // TODO: render our border
     for_each_child([&](auto& child) {
+        if (child.is_box() && to<LayoutBox>(child).stacking_context())
+            return;
         child.render(context);
     });
 }