Rescale 1088-row videos to avoid showing trailing garbage.

Refs #11358.  1080i videos may present themselves as 1088 rows
(divisible by 16).  The HD-PVR in particular may contain garbage in
those last rows.  Specifically, the last two rows may form an annoying
green line.  We add a scaling pass to shift the bottom 4 rows (4 to be
on the safe side) off the bottom of the display area.