LibWeb: Run IntersectionObserver steps only when needed

Instead of updating IOs in every iteration of the HTML event loop,
we now only do it after a relayout, or after the viewport changes.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -1106,6 +1106,9 @@ void Document::update_layout()
 
     m_needs_layout = false;
     m_layout_update_timer->stop();
+
+    // OPTIMIZATION: We do this here instead of in HTML::EventLoop::process() to avoid redundant work.
+    run_the_update_intersection_observations_steps(HighResolutionTime::unsafe_shared_current_time());
 }
 
 [[nodiscard]] static Element::RequiredInvalidationAfterStyleChange update_style_recursively(Node& node)