Fixes issue2632 if interl=1 is used or the automatic interlace detection is enabled
and works. This has the advantage compared to the patch in issue2632 that it causes
no speed loss and it also works when scaling is used. The disadvantage is that
interlacing autodetection does not yet work very well it seems.
This is the same method mplayer uses
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>