LibWeb: Batch processing of successfully downloaded images

Before this change, we would process each image as it finished
downloading. This often led to a situation where we'd decode 1 image,
schedule a layout, do the layout, then decode another image, schedule
a layout, do the layout, etc. Basically decoding and layouts would get
interleaved even though we had multiple images fetched and ready for
decoding.

This patch adds a simple BatchingDispatcher thingy that HTMLImageElement
uses to batch the handling of successful fetches.

With this, the number of layouts while loading https://shopify.com/ goes
from 48 to 6, and the page loads noticeably faster. :^)

Userland/Libraries/LibWeb/HTML/HTMLImageElement.cpp

@@ -262,6 +262,40 @@ bool HTMLImageElement::uses_srcset_or_picture() const
     return has_attribute(HTML::AttributeNames::srcset) || (parent() && is<HTMLPictureElement>(*parent()));
 }
 
+// We batch handling of successfully fetched images to avoid interleaving 1 image, 1 layout, 1 image, 1 layout, etc.
+// The processing timer is 1ms instead of 0ms, since layout is driven by a 0ms timer, and if we use 0ms here,
+// the event loop will process them in insertion order. This is a bit of a hack, but it works.
+struct BatchingDispatcher {
+public:
+    BatchingDispatcher()
+        : m_timer(Core::Timer::create_single_shot(1, [this] { process(); }).release_value_but_fixme_should_propagate_errors())
+    {
+    }
+
+    void enqueue(JS::SafeFunction<void()> callback)
+    {
+        m_queue.append(move(callback));
+        m_timer->restart();
+    }
+
+private:
+    void process()
+    {
+        auto queue = move(m_queue);
+        for (auto& callback : queue)
+            callback();
+    }
+
+    NonnullRefPtr<Core::Timer> m_timer;
+    Vector<JS::SafeFunction<void()>> m_queue;
+};
+
+static BatchingDispatcher& batching_dispatcher()
+{
+    static BatchingDispatcher dispatcher;
+    return dispatcher;
+}
+
 // https://html.spec.whatwg.org/multipage/images.html#update-the-image-data
 ErrorOr<void> HTMLImageElement::update_the_image_data(bool restart_animations, bool maybe_omit_events)
 {
@@ -473,6 +507,7 @@ after_step_7:
 
         image_request->add_callbacks(
             [this, image_request, maybe_omit_events, url_string, previous_url]() {
+                batching_dispatcher().enqueue([this, image_request, maybe_omit_events, url_string, previous_url] {
                     VERIFY(image_request->shared_image_request());
                     auto image_data = image_request->shared_image_request()->image_data();
                     image_request->set_image_data(image_data);
@@ -511,6 +546,7 @@ after_step_7:
                     }
 
                     m_load_event_delayer.clear();
+                });
             },
             [this, image_request, maybe_omit_events, url_string, previous_url, selected_source]() {
                 // The image data is not in a supported file format;