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/CMakeLists.txt

@@ -94,6 +94,7 @@ set(SOURCES
     Layout/LayoutWidget.cpp
     Layout/LineBox.cpp
     Layout/LineBoxFragment.cpp
+    Layout/StackingContext.cpp
     LayoutTreeModel.cpp
     Loader/FrameLoader.cpp
     Loader/ImageLoader.cpp