Noop on clicking both mouse buttons

[brisvag]

Small change to 3D cameras mouse event callback to prevent firing a million warnings when accidentally pressing both right and left mouse buttons: napari/napari#3518.

I'm not sure if we prefer a noop (as I did in this PR) or just ignoring the second click, though the second option requires a bit more refactoring I think.

[djhoese]

Where does the warning come from?

[brisvag]

Something about how self._event_value is used and updated gets thrown off by multiple clicks, so that self._event_value[0] is None when running _update_rotation, rather than a float.

  File "/home/brisvag/git/vispy/vispy/scene/cameras/perspective.py", line 225, in viewbox_mouse_event
    self._update_rotation(event)
  File "/home/brisvag/git/vispy/vispy/scene/cameras/turntable.py", line 136, in _update_rotation
    self.elevation = self._event_value[1] + (p2 - p1)[1] * 0.5
TypeError: unsupported operand type(s) for +: 'NoneType' and 'float'

[djhoese]

Do we have any real tests for the cameras where this could be added?

[brisvag]

Not sure how to add fake clicks or at least mouse events to tests.

[djhoese]

Eh don't worry about it. Thanks.