LibWeb: Only invalidate stacking context tree for opacity/z-index change

I came across some websites that change an elements CSS "opacity" in
their :hover selectors. That caused us to relayout on hover, which we'd
like to avoid.

With this patch, we now check if a property only affects the stacking
context tree, and if nothing layout-affecting has changed, we only
invalidate the stacking context tree, causing it to be rebuilt on next
paint or hit test.

This makes :hover { opacity: ... } rules much faster. :^)

Userland/Libraries/LibWeb/DOM/Document.h

@@ -148,6 +148,7 @@ public:
     void set_needs_layout();
 
     void invalidate_layout();
+    void invalidate_stacking_context_tree();
 
     virtual bool is_child_allowed(const Node&) const override;