理解SetWindowOrg,SetViewportOrg,SetWindowExt,SetViewportExt
http://wenku.baidu.com/view/da0e8692daef5ef7ba0d3cfb.html
VC++技术内幕(第四版)笔记--SetWindowExt和SetViewportExt
http://www.cnblogs.com/sxlfybb/archive/2006/06/13/424770.html