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variable: $zrot -- rotate brain in window plane
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keyboard equivalent: [Ctrl]-L/R arrow keys
direct manip. equiv.: none

Rotates brain surface in the plane of the window
(i.e., around the z-axis) in degrees (positive is
counter-clockwise (convention on 2D Cartesian
axes).

In a tcl rendering script, this rotation can be
achieved with:

  rotate_brain_z <deg>
  redraw
