To solve the problem of rendering blocking for vsync "storm".

If a frame cost time take longer than our frame interval, follow frames will continous enter vsync blocking problem, and need a long time to self recover. 
For more discussion, see this article http://www.ananseproductions.com/game-loops-on-ios/
This code will solve this problem, if one frame take over time, next frame will be skipped, and follow frames will be triggered in right time.