Expose `avformat_seek_file`

[AlaaAlHallaq]

hello, I have been using ffmpeg for couple of years in my apps, and your library is extremely useful to simplify accessing ffmpeg functionalities.
I need you to expose
avformat_seek_file
since
av_seek_frame
has some issues seeking most of the files I tested, I have a fork of your project, and used avformat_seek_file, and it worked for most of my test files.

[gregcotten]

av_seek_frame works just fine for every file I have ever thrown at it - what issues are you having?

Of course we could expose support for this method, but just curious what kind of issues you are facing with the current seeking method.

[AlaaAlHallaq]

@gregcotten simply most of my test files did not work to seek through the file using av_seek_frame, forking this library and exposing avformat_seek_file, did the trick, noting that I have custom IO using AVIOContext.

[AlaaAlHallaq]

@gregcotten ?

[gregcotten]

Don't have time to work on it at the moment, but I see your fork - could you make a PR?