[mythtv] underscan OSD only

[Jim Paris]

My TV overscans about 5-7% on each border.  I am connecting to it
through VGA and if I display video full-screen, the sizing is perfect.
For the GUI, I've set the dimensions to match what the visible area.
No problems there.  For TV playback, I have two options:

- Set GuiSizeForTV with overscan percentages = 6.  Then,
  the OSD fits within the viewable area, but the video size
  is slightly off due to rounding errors etc, and it's being
  scaled unnecessarily.
- Disable GuiSizeForTV; then the video is fullscreen and displays
  perfectly but the OSD also extends into the overscanned area.

I want to add a third option which will still display the video
fullscreen but shrink the OSD by 6%, say, "Over/Underscan OSD Only".
All the dispx/dispw/imgx/imgw/XJ_width/winw/etc stuff is confusing;
rather than staring at the code for too long, could someone point me
to the appropriate place to set this sizing?  (Or smack me if I'm
missing something obvious?)

-jim