该函数将显示GUI选择正方形或长方形的部分

代码说明：

This functions displays GUI for selecting square or rectangular part of the input image IMG. To perform selection user must click mouse twice: at two corners of the selection area. User can change the shape at any moment, even when first point is set, unless it is not forbidden by additional parameters. Use also cam change the way the selection area is calculated from the two selected points. Depending on the combination of the shape and mode it could be