[mythtv] Overscan doing weird things (wrapping) with bob deinterlace

[Kyle Rose]

FWIW, I've (incorrectly) had non-zero overscan and scan displacement on
my desktop frontend for a long time; since the videoout_xv.cpp changes,
the top X lines of each frame have appeared at the bottom of the display
window, and everything else has been moved up by X lines.  This effect
disappears when I zero out the overscan stuff.

Strange thing is that this effect only appears with bob, by which I
probably mean only double framerate deinterlacers.  Just FYI, if you
encounter a vertical wrapping problem, this is probably the issue.

Cheers,
Kyle