LibWeb: Make CSS layout lazier

Instead of doing layout synchronously whenever something changes,
we now use a basic event loop timer to defer and coalesce relayouts.

If you did something that requires a relayout of the page, make sure
to call Document::set_needs_layout() and it will get coalesced with all
the other layout updates.

There's lots of room for improvement here, but this already makes many
web pages significantly snappier. :^)

Also, note that this exposes a number of layout bugs where we have been
relying on multiple relayouts to calculate the correct dimensions for
things. Now that we only do a single layout in many cases, these kind of
problems are much more noticeable. That should also make them easier to
figure out and fix. :^)

Userland/Libraries/LibWeb/Page/BrowsingContext.cpp

@@ -82,6 +82,8 @@ void BrowsingContext::set_viewport_rect(Gfx::IntRect const& rect)
 
     if (m_size != rect.size()) {
         m_size = rect.size();
+        if (auto* document = active_document())
+            document->set_needs_layout();
         did_change = true;
     }
 
@@ -105,6 +107,9 @@ void BrowsingContext::set_size(Gfx::IntSize const& size)
         return;
     m_size = size;
 
+    if (auto* document = active_document())
+        document->set_needs_layout();
+
     for (auto* client : m_viewport_clients)
         client->browsing_context_did_set_viewport_rect(viewport_rect());