This is a prerequisite for making any encoder that uses
ff_mpv_encode_init() init-threadsafe; it already makes the AMV,
the MJPEG and the MPEG-1/2 encoders init-threadsafe.
Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>