vc1dec: Update 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: Martin Storsjö <martin@martin.st>
