Original commit:
commit 2473a45c
Author: Janne Grunau <janne-libav@jannau.net>
Date:   Wed Jan 18 10:53:41 2012 +0100

    threads: change the default for threads back to 1

    Using threaded decoding by default breaks backward compatibility if
    AVHWAccel is used or if an appliction sets threadunsafe callbacks.
    Avconv and avplay still use -threads auto if not specified.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>