https://stackoverflow.com/questions/44447504/do-not-understand-region-class-very-well
说明:绘制的矩形不能正确显示
ClipRegion 不同于 ClipRect,绘制的起点坐标是屏幕的起点坐标,所以你会觉得没有正确显示。
region API http://blog.csdn.net/lonelyroamer/article/details/8349601
Region.OP http://blog.csdn.net/eyishion/article/details/53728913