-------------------------------------------
tickbox variable: $zoomflag -- toggle zoom all3views
-------------------------------------------
keyboard equiv: <none>

The "z" tickbox is only visible when "all3views"
is ticked.  This doubles the magnification of
coronal, horizontal, and sagittal views and moves
cursor to match unzoomed location (currently
selected slice plane indicated by max intens
projection).

Clicking cursor in any plane correctly updates
other two slice planes (struct and overlay) as
with unzoomed all3(views).

This is a quick hack for easier viewing of
brainstem activations.  It has shortcomings:

 1) it is not possible to pan the individual
    centered slice views (only the central
    +/- 128mm of the FOV is accessible)

 2) slice plane slider numbers (and Talaiarch
    coords) change to centered units of 1/2 mm
    for 1mm^3 data (or 0.25mm for 0.5mm^3 data)

 3) because of 2), intrinsic and Talairach coord
    entries, and SEND PNT and GOTO PNT are
    disabled during zoom.  To use these
    functions, unzoom, use them, then re-zoom.
