Snapping 3D rotation angles with Control key

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.