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.
It does, but it's using an SDL(2) event loop to handle doing things on time in the main thread loop. I'm not sure how easy it would be to do with Ogre here.