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tickbox variable: $labelmaskflag -- toggle mask w/3D ROI
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keyboard equiv: <none>

The "m" tickbox (next to the "roi" tickbox)
toggles masking of overlay data with 3D ROI.
This requires that an ROI BRIK has first been
read in with the "R" button on the "roi:" line.

In places where the ROI mask is not 0 (areas that
are white when "roi" is ticked), the overlay data
is displayed.

In places where the mask=0, the overlay data is
masked (only the underlay is visible).

N.B.: the overlay data will be covered by white
if "roi" is also ticked.

N.B.: this does *not* control application of stat
mask.  For that, use tickbox just to left of
"mskn:"/"mskup:"
