FIX: [h264_mp4toannexb] issue when sps/pps are already in the bistream

This fix an issue in AnnexB conversion where "hiccups" are to be seen if sps/pps are already in the stream.

It's a follow-up to http://ffmpeg.org/pipermail/ffmpeg-devel/2014-March/155911.html , which solved most of the issues but not all.
See https://github.com/xbmc/xbmc/pull/5042 for the XBMC discussion.

My best guess for now is that https://github.com/FFmpeg/FFmpeg/commit/289b149cecb381522cc9ccdf382825330169c655#diff-0e332ff3db7b093bfee7dda7ed82ce19 is flawed if non-idr pics have sps/pps of their own. Then ffmpeg adds extradata to non-idr pics, too. 

If the stream already has sps/pps of their own, I don't think there is an added value to re-add those from extradata, so this patch just don't prepend if they're already present. 

Sample: http://www.semperpax.be/owncloud/public.php?service=files&t=a28e4f3e2305ba08837c45f42c4a3662