Small change to make spritemaps animate the same in fixed timestep mode

Right now spritemaps animate at a different rate in fixed timestep mode. To make them animate similarly to in non-fixed timestep mode, you have to give animations very low framerate values (<1). This change makes the timer in fixed timestep mode work similar to how it works in non-fixed timestep mode by dividing the value of _anim.frameRate by HXP.frameRate.

Also, see this thread: http://forum.haxepunk.com/index.php?topic=400.msg1106