Closed #2429: Fixed a bug of cc.Layer's bake function.
Merge branch 'develop' of https://github.com/cocos2d/cocos2d-html5 into Iss2429_LayerBake
Closed #2429: math.ceil to 0| (x + 0.5)
Merge pull request #2430 from dingpinglv/Iss2429_LayerBake

Closed #2429: Fixed a bug of cc.Layer's bake function.