3D axes rotation now supports snapping to fixed angular increments
when holding the Control key during mouse rotation.
The snap step size is controlled by the new
axes3d.snap_rotation rcParam (default: 5.0 degrees).
Setting it to 0 disables snapping.
For example:
mpl.rcParams["axes3d.snap_rotation"] = 10
will snap elevation, azimuth, and roll angles to multiples of 10 degrees while rotating with the mouse.