title: Using NSTimer to get accurate timings
body: <p>I need to run my game loop with very accurate timing. I am trying to use NSTimer to do this, and I am getting ok results, but there is a bit of drift.
  When NSTimer fires does the next time event start counting when the handler finishes or does it start counting straight away. 
  If the former is it reasonable for me to use setFireDate to try to offset the next timer firing - when I tried this things seemed worse.</p>
  
  <p>My timer is set to fire every 44ms and I would like to stop it drifting by more than 20ms. If it does drift I would like to correct it for the next fire.
  Is this a reasonable thing to try and do with NSTimer?</p>