avcodec/vc1: change the internal ordering of blocks within a macroblock

The overlap filter needs to cover a full macroblock vertical edge when the
FIELDTX value for two neighbouring macroblocks is not equal. By changing
the internal ordering of the blocks from row major to column major, we do
not need to reinterlace a FIELDTX coded macroblock before running the overlap
filter.
Signed-off-by: Jerome Borsboom <jerome.borsboom@carpalis.nl>