When using hardware accelerated decoding for multi-slice field interlaced pictures,
only the first slice was decoded. This patch adds the neccesary looping over the
remaining slices that may exist in field interlaced pictures. Additionally, we align
the calculation of mby_start for the second field with the method given in VC-1 spec.
Signed-off-by: Jerome Borsboom <jerome.borsboom@carpalis.nl>