Refactor add_ffmpeg_audio.

Simplify, set bitrate to 0 - scan will then call decavcodecaBSInfo,
decode some audio and set the other input parameters, including samples_per_frame.

decavcodec and scan fixes by John Stebbins. Thanks!



git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5424 b64f7644-9d1e-0410-96f1-a4d463321fa5