vc1dec: Fix destination pointers in context for fields

This replaces a large number of checks for the second field by
fixing the pointers when they are setup

This should also fix I/BI field pictures

Changes checksums for vc1_sa10143, the file becomes slightly closer
to what the reference decoder outputs

Based on "vc1dec: the second field is written wrong to the picture"
         by Sebastian Sandberg <sebastiand.sandberg@gmail.com>

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>