Interactive render crosshairs for the Multi-Planar A+C+S+R menu
item in the Display item shows 2D slices and a volume rendering.
Clicking on any of the four images changes the cross-hair position
across all views.
The Option button includes Find Edges command that provides a
Difference of Gaussians. Also available in scripts via dog()
function.
Measuring tool (option-drag across slices) shows sample along
line graph.
