Optimize CCSpriteFrame's init performance

[WanderWang]

reduce cc.rect() function call in ctor() to optimize performance because of cc.SpriteFrame.initWithTexture() will call the function duplicately

[dingpinglv]

Hi Ze,

I agree with your refactoring, but I think you should add a condition to setRectInPixels and setRect of CCSpriteFrame. It will throw an error when some developers write codes like this:

var aSpriteFrame = new cc.SpriteFrame();
aSpriteFrame.setRect(cc.rect(10,10,20,20)); // it throws an error.

Although, almost of all developers like to call cc.SpriteFrame.create() to create a cc.SpriteFrame object, but we should add some conditions to avoid errors.

Thanks for your refactoring.
David