更换背景图：
auto background1 = Sprite::create("original.png");
Texture2D * pic = Director::getInstance()->getTextureCache()->addImage("now.png");
background->setTexture(pic);
更换背景图：
auto background1 = Sprite::create("original.png");
Texture2D * pic = Director::getInstance()->getTextureCache()->addImage("now.png");
background->setTexture(pic);