LibWeb: Schedule input event processing on HTML event loop

Our existing coalescing mechanism for input events didn't prevent
multiple mousemove/mousewheel events from being processed between paint
cycles. Since handling these events can trigger style & layout updates
solely for hit-testing purposes, we might end up doing work that won't
be observable by a user and could be avoided by shceduling input events
processing to happen right before painting the next frame.

Libraries/LibWeb/HTML/EventLoop/EventLoop.h

@@ -95,6 +95,7 @@ private:
 
     virtual void visit_edges(Visitor&) override;
 
+    void process_input_events() const;
     void update_the_rendering();
 
     Type m_type { Type::Window };