[Update] support "clip input region"

[yanang007]

我注意到目前拼接出来的图像是所给经纬度区域包含的所有块直接拼接，会一定程度上包含给定区域之外的部分，因此希望有一个选项能支持裁剪输出结果至只包含用户指定区域的图像。 该pr实现了从拼接的结果图中裁剪掉多余的区域，得到匹配指定范围的图像的功能，通过-clip参数指定，为保持兼容性默认为false。 如果有其它需要修改的部分欢迎随时通知我。: )

I noticed that current output image is a direct combination of all blocks involved, which leads to redundent area that may not be expected. Thus any option to clip exactly specified region from output may be useful. This pr implemented support for clipping the exact region from combined image, and added a related option -clip which defaults to false for compatibility. Please let me know if any improvement is required. :)