Copied verbatim from ?rgl.setMouseCallbacks for rgl version 0.92.892 Mouse
button 2 is right and button 3 is middle (accessed by meta/alt key)
Note that sometimes (parts of) objects seem to disappear after panning and
zooming. The example in rgl.setMouseCallbacks
from which this
is copied includes a note that "this doesn't play well with rescaling"