-------------------------------------------
tickbox variable: $overlayflag -- toggle funct overlay
-------------------------------------------
default: 0 (FALSE)

The "ovlag" tickbox (just right of center)
toggles visibility of a data overlay.

When "ovlay" is ticked ($overlayflag is 1), the
background surface colors are changed to grays,
and functional data (from the .val[.val2] fields)
is overlaid in color on the surface.

If curvature has been loaded, and radio button to
right of "Col:" is set to "curv", the curv scale
changes from red/green to light/dark gray.  If
the "none" radio button is selected, there is a
single shade of gray.

The brightness ratio of the grays is adjustable
by $cvfact (def=1.3) and the single or darker
gray color is set by $offset (def=0.3) (unmarked
entries just to right).
