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. :^)

Meta/Lagom/Tools/CodeGenerators/LibWeb/Generate_CSS_PropertyID_cpp.cpp

@@ -155,6 +155,34 @@ bool property_affects_layout(PropertyID property_id)
     }
 }
 
+bool property_affects_stacking_context(PropertyID property_id)
+{
+    switch (property_id) {
+)~~~");
+
+    properties.for_each_member([&](auto& name, auto& value) {
+        VERIFY(value.is_object());
+
+        bool affects_layout = true;
+        if (value.as_object().has("affects-stacking-context"))
+            affects_layout = value.as_object().get("affects-stacking-context").to_bool();
+
+        if (affects_layout) {
+            auto member_generator = generator.fork();
+            member_generator.set("name:titlecase", title_casify(name));
+            member_generator.append(R"~~~(
+    case PropertyID::@name:titlecase@:
+)~~~");
+        }
+    });
+
+    generator.append(R"~~~(
+        return true;
+    default:
+        return false;
+    }
+}
+
 NonnullRefPtr<StyleValue> property_initial_value(PropertyID property_id)
 {
     static Array<RefPtr<StyleValue>, to_underlying(last_property_id) + 1> initial_values;

Meta/Lagom/Tools/CodeGenerators/LibWeb/Generate_CSS_PropertyID_h.cpp

@@ -90,6 +90,7 @@ bool property_accepts_value(PropertyID, StyleValue&);
 size_t property_maximum_value_count(PropertyID);
 
 bool property_affects_layout(PropertyID);
+bool property_affects_stacking_context(PropertyID);
 
 constexpr PropertyID first_property_id = PropertyID::@first_property_id@;
 constexpr PropertyID last_property_id = PropertyID::@last_property_id@;