requestAnimationFrame support

[andygill]

Support accessing the requestAnimationFrame concepts from blank canvas.

• http://creativejs.com/resources/requestanimationframe/
• http://www.html5rocks.com/en/tutorials/speed/animations/
• http://updates.html5rocks.com/2012/05/requestAnimationFrame-API-now-with-sub-millisecond-precision

[andygill]

Do we use

requestAnimationFrame :: Canvas a -> Canvas a

or

requestAnimationFrame : Canvas ()

or

requestAnimationFrame :: Context -> Canvas a -> IO a

[andygill]

There is another possible:

requestAnimationFrame :: Canvas Float 

where Float is the time in seconds since the last requestAnimationFrame, or a get-time-of-day counter.

[nomeata]

I could use this function for my current project.