Support cropping / region of interest

[Breakthrough]

Processing only a sub-set of a video frame should be possible. This can greatly improve performance and detection accuracy in some cases.

To avoid any issues with cropping or output, both save-images and split-video should still output the full video. Cropping of the output can be considered in the future, but should be avoided if possible and done by post-processing the output of PySceneDetect. The same applies to non-rectangular ROIs or overlapping regions.

This was originally brought up in #147. This is also a feature in PyVfxShotDetect [1], so we can consider implementing something similar by creating a wrapper for VideoStream.

[1] https://github.com/ThomasWeckenmann/PyVfxShotDetect/blob/master/scenedetect/bbox_video_stream.py