[Request] Expose ``AVFormatContext::start_time`` in the cpp API

[moi15moi]

It would be usefull to expose AVFormatContext::start_time to user.
This is usefull for tool like aegisub which want to emulate how mpv set the timestamps.

[myrsloik]

How is AVFormatContext::start_time different from the individual track start times? Why do I want to emulate mpv timestamp?

[moi15moi]

How is AVFormatContext::start_time different from the individual track start times?

I'm not totally sure how AVFormatContext::start_time is set. As said here, it seems ffmpeg use the lowest track start_time (any type of tracks, subs, audio, video)

Why do I want to emulate mpv timestamp?

As said here, for .m2ts file, in general, the timestamps doesn't start at time 0.
When editing subtitle, it doesn't make sense to use the time 1:10:00.000 for the first frame, but it also doesn't make sense to just shift the timestamps to 0.