Support stereo as experimental feature in mmf / Yamaha SMAF.

Fixes ticket #1085.
doc/ffmpeg-resampler: Try to document async

Improvements welcome!

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
add note on windows pipe

Signed-off-by: rogerdpack <rogerpack2005@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>