The original implementation only sent out notifications when there was
something being drawn on screen. If nothing was going on, we'd get too
lazy and just not notify display links.
This obviously break requestAnimationFrame(), so now we just drive the
DisplayLinks at 60 fps no matter what. :^)
This patch adds GUI::DisplayLink, a mechanism for registering callbacks
that will fire at the display refresh rate.
Note that we don't actually know the screen refresh rate, but this is
instead completely driven by WindowServer's compositing timer. For all
current intents and purposes it does the job well enough. :^)
