Files produced by windows media center contain meaningless mpeg1 sequence
header. The mpeg2 decoder detects the presence mpeg1 sequence header start
codes and attempts to decode the stream as mpeg1. (This problem introduced
in 73a2d16b.)

Fixes ticket #3601.
Signed-off-by: Peter Ross <pross@xvid.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>