Jack transport control support

Expected behavior of the wanted feature

I had been wanting to watch a commentary that is synchronized (with an offset and slides) to a film. There's very little in the way of software to support this - people are generally expected to ensure their players are synced up between the commentary and the film.

On there other hand, there exists an application called xjadeo. The purpose of xjadeo is to manage synchronized video playback for audio dubbing or installations (large video displays, etc...). xjadeo strictly supports video, not audio, playback and is very particular about what files it opens (it seems to be OK with mjpeg, and some mp4 streams). Essentially, the only useful features that xjadeo has over mpv is as follows:

1. It can be controlled using the jack transport system (time code, play, pause, etc...)
2. Individual instances can have their position offset by a number of frames from the transport time

Adding JACK transport support to mpv would enable personal use cases such as this, as well as greater simplicity for those that wish to create large synchronized displays with audio (as this would eliminate the need to synchronize audio on top of the video).

The desired behaviour is as follows:

1. MPV can be controlled by the JACK transport
2. An MPV instances playback position can be controlled by the JACK transport time
3. An MPV instance can have its position offset from the JACK transport time, forward or backward.

Start by researching the JACK transport system and how mpv currently handles playback control and positioning. The issue does not name files or tests, so locating the relevant entry points will require project familiarity. Done means mpv responds to JACK play, pause, and time controls, with configurable forward or backward position offsets.