Fix bug that was causing one sec. of audio to be dropped on many ffmpeg files. Proble...

Fix bug that was causing one sec. of audio to be dropped on many ffmpeg files. Problem is that audio & video can be interleaved in any order but we drop everything up to the first video frame. Since ffmpeg returns a second of audio per read on an audio stream, if audio started before video we lose the first second of it. Changed to allow either audio or video to signal start 'reader'.

git-svn-id: svn://localhost/HandBrake/trunk@2831 b64f7644-9d1e-0410-96f1-a4d463321fa5