Vertical lookat vector fails

[tomvanmele]

Describe the bug

If the lookat vector is vertical, the viewer failts.

To Reproduce

Steps to reproduce the behavior:

viewer = App()
viewer.view.camera.position = [0, 0, 10]
viewer.view.camera.look_at([0, 0, 0])
viewer.run()

Expected behavior

A clear and concise description of what you expected to happen.

Screenshots

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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Python version [e.g. 2.7]
• Python package manager [e.g. macports, pip, conda]

Additional context

Add any other context about the problem here.

[Licini]

@tomvanmele Fixed now.