Use correct scaling table for bwd-pred MVs in second B-field

When scaling backward predicted MVs in second B-field, the scaling table is
opposite of that for P field pictures; i.e. first field P table will be used as
second field B table and second field P table will be used as first field B
table. This is not documented in the spec, but exists in the ref. decoder.

This fixes SA10139.

Signed-off-by: Martin Storsjö <martin@martin.st>