Supertx fix for 422 colour subsampling

Fixes assertion for football_422_4sif.y4m when supertx, var_tx and
ext_tx are all enabled. Problem was after subsampling, the u and v
blocks being encoded were no longer square.

Change-Id: Ie626f30a2e64538d33343a26d5124a79a6f2b985
1 file changed