Feature: Add option to use embedded cover as thumbnail (for video files)

[camoz]

Some video container formats, like mp4 and mkv, support adding a cover to the video file. It would be nice to have an option to use this embedded cover as a thumbnail, instead of generating a new one.

My personal use case: I download youtube videos using yt-dlp, which has an option to embed covers (--embed-thumbnail). I'd like to browse through them using qimgv, using the original thumbnail (which is the same as on youtube).

Btw, a cover can be added to an mp4 file like this:

ffmpeg -i video.mp4 -i image.png -map 1 -map 0 -c copy -disposition:0 attached_pic out.mp4