For B field pictures, the spec says,
> The prediction shall be made from the field of the same parity as the field being predicted.
I did it.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 8b154cb3e90a3e599cadf477d815a9854b7bb4e1)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>