CCDirector.secondsPerFrame not updated.

[slembcke]

http://www.cocos2d-iphone.org/forums/topic/ccdirector-shareddirector-secondsperframe-returns-value-of-zero/

[CCDirector calculateMPF] is only called when the stats label is shown. Should just call it all the time perhaps? Either that or remove the CCDirector.secondsPerFrame property.

[slembcke]

This is going to end up being rewritten for 4.0. Just need to make sure we test it.