Don't allow to zoom "through" the image

[gwhitney]

In ui2, currently 2-dimensional visualizations using P5GLVisualizer implement zoom just by moving the camera toward the plotting plane (z=0). But at the moment, after a finite number of steps, the camera actually moves beyond that plane, and thereafter nothing is displayed. This should not be allowed. Perhaps zoom should multiply/divide the distance to the plotting plane by something, rather than add/subtract from it.