[FFmpegFrameGrabber] improved precision of seek to audio or video frames

[anotherche]

Precision of seek to audio or video frames is improved.
Done after the discussion on issue #975.
New code of setTimestamp now seeks to the frame just containing the desired timestamp if a specific frame type (video or audio) is requested explicitly (for example, if setVideoTimestamp or setVideoTimestamp is called).
The old code mostly returned frames next to the frames which should be actually returned (the frames containing the desired timestamps).
Additionally, the new code is ~2x faster when video or audio frames are explicitly requested.
A bug is fixed in getAudioFrameRate method as well.

[saudet]

Can you make all the new code active only for the case when frameTypesToSeek != null?

[anotherche]

All the new seek code is enclosed now within two frameTypesToSeek != null blocks.

[anotherche]

There are only two variables, ts0 and early_ts, which are declared and used outside. It is because I need different start time of the stream ts0, and different timestamp for avformat_seek_file method.