Underlay metrics.

I've added this at 0,0, instead of CENTER, because it's usually a BGA;
it's more natural to specify absolute coordinates in a top-level BGA than
coordinates relative to the center.  I'll change Overlay over later when I
have time to test it.

stepmania/Themes/default/metrics.ini

@@ -2412,6 +2412,10 @@ OverlayX=SCREEN_CENTER_X
 OverlayY=SCREEN_CENTER_Y
 OverlayOnCommand=
 OverlayOffCommand=
+UnderlayX=0
+UnderlayY=0
+UnderlayOnCommand=
+UnderlayOffCommand=
 CodeNames=
 
 [MenuTimer]