avcodec/vc1: fix mquant calculation for interlace field pictures

For interlace field pictures s->mb_height indicates the height of the full
picture in MBs, i.e. the two fields combined. A single field is half this
size. When calculating mquant for interlace field pictures, the bottom edge
is the last MB row of the field.

Signed-off-by: Jerome Borsboom <jerome.borsboom@carpalis.nl>