说明：  一个绘制背景类，把背景分为左中右或者上中下三部分，通过对中间部分图片的缩放来实现整张背景的缩放，自动透明背景
(A drawing class background, the background is divided into left and right or on the lower three parts, the middle part of the image through the zoom to achieve the entire background of the zoom, automatic and transparent background)