Runs ffmpeg cropdetect (per-frame reset, rows and columns whose mean luma is at ...
https://ton.halowerk.com/v1/letterbox-detectOverview
Runs ffmpeg cropdetect (per-frame reset, rows and columns whose mean luma is at or below black_level count as black) on the keyframes of the selected range, or on evenly spaced frames when there are fewer keyframes than samples. Frames that are black as a whole are ignored; the union of the remaining picture areas is the content area, so no frame loses content.