对用户自定义的多边形进行裁剪，裁剪框也是由用户自定义

polygonClip功能，地理信息系统科学二次开发部分功能珍贵源代码，分享给做地理信息系统二次开发的朋友(Geographic information system science two development part, the function is precious, the source code, shares to make the geography information system two development friend)