LibWeb: Add basic support for requestAnimationFrame()

We now support rAF, driven by GUI::DisplayLink callbacks. It's a bit
strange how we keep registering new callbacks over and over.
That's something we can definitely optimize.

This allows you to update animations/whatever without doing it more
often than the browser can display.

Libraries/LibWeb/DOM/CanvasRenderingContext2D.h

@@ -28,6 +28,8 @@ public:
 private:
     explicit CanvasRenderingContext2D(HTMLCanvasElement&);
 
+    void did_draw(const Gfx::Rect&);
+
     OwnPtr<Gfx::Painter> painter();
 
     WeakPtr<HTMLCanvasElement> m_element;