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.
Repeating the sample is probably the best option, since using silence has the potential to add clicks and pops. IMO, the conversion should happen in audio_decode_frame, right after it decodes a packet. I've actually made a redone version of this code as an OpenAL Soft example player, using swresample, which you can see here:

http://repo.or.cz/w/openal-soft.git/blob/HEAD:/examples/alffplay.c#l374