VC1: Fix first/last row checks with slices

In some places 0/mb_height were used in place of start_mb_y/end_mb_y.

Fixes SA00049, SA00058, SA10091, SA10097, SA10131, SA20021, SA30030

Improves PSNR in SA00054, SA00059, SA00060, SA10096, SA10098, SA20022,
SA30031, SA30032, SA40012, SA40013

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
