Implement PTS fixer for RTSP adapter

[bwsw]

Sometimes, RTSP does not produce consistent PTS, so the module executes the end-of-stream behavior for such situations. We can introduce a feature allowing users to fix PTS. Hopefully, it can be done easily with FFmpeg properties, otherwise we need to fix them manually on our side.

[bwsw]

Use only positive deltas and the previous PTS as a base. Maybe, we need to improve DTS as well.

Also, please check if can be done with extra argument to ffmpeg in ffmpeg_input. If can be done, please decline the task.