This function
sets the location of the viewer relative to the scene, after the model transformations
(scaling, rotation) have been done, but before lighting or projection have been
applied. (See par3d for details on the rendering pipeline.)
The coordinate system is a slightly strange one: the X coordinate
moves the observer location from left to right, and the Y coordinate moves up
and down. The Z coordinate changes the depth from the viewer. All are measured
relative to the center of the bounding box (par("bbox")) of the subscene.
The observer always looks in the positive Z direction
after the model rotation have been done. The coordinates are in post-scaling
units.