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/Forward.h

@@ -64,6 +64,7 @@ class RenderingContext;
 class Resource;
 class ResourceLoader;
 class Selector;
+class StackingContext;
 class StyleResolver;
 class StyleRule;
 class StyleSheet;