Ffmpeg update

Works, but uses 32bit output and only supports stereo.  <snip>

EDIT: FFMpeg distributed for windows (http://ffmpeg.zeranoe.com/builds/) do not provide libavresample, while some Linux systems do not support libswresample.  Need to compile ffmpeg from source if libavresample is required, which will in turn involve setting up MinGW/Msys dev env.

This PR should still resolve bug 1733 but would appreciate feedback from linux/osx users.  Also welcome code review comments.
If I remember correctly, Chris mentioned that this could be changed to emit silence samples instead of repeating samples. That would make the conversion unnecessary.