Lossless keyframe manipulation

[mifi]

Features that involve copying/moving keyframe losslessly. Note that I have no idea how to implement this with ffmpeg, but I'll leave it open for ideas.

Lossless masking

See #1895

Pick A Keyframe To Be The First Frame

Discussed in #1259

^{Originally posted by rd-sf August 3, 2022}
Has this been requested? The ability to stop the timeline on a keyframe, hit a button, and have LC pull that keyframe and copy it to the beginning of the video stream. This would help folks who use players that use the very first frame as the thumbnail, like iOS does. Many Many Thanks for making and maintaining Lossless Cut.

This feature is a bit of a "hack", but if it's something that many people find interesting, it could be worth looking into.