图像局部放大功能。包括主要的三个消息处理函数，由鼠标拖动画出需放大区域，松开即将图像局部放大到整个客户区，能够实现层层放大。-enlarge images partial function. Including the three main message processing function, the mouse scrolling painted by the need to enlarge the regional, Loosen the forthcoming local enlarg