Fix iOS 4.3 JPEG loading (darken picture, alpha issue)

This code fixes JPEG files not being loaded properly under iOS 4.3.
Clean PR this time :)

Please refer to :
https://github.com/cocos2d/cocos2d-x/pull/1942
http://www.cocos2d-x.org/boards/6/topics/17929
http://stackoverflow.com/questions/13254446/cocos2d-x-jpg-image-becomes-dark