Fix omnidir::undistortPoints

[rgolovanov]

The relevant bug was reported in #1612

The _xi was erroneously applied at points re-projection to camera plane.
_xi parameter was already taken in use while projection of points to unit sphere.

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[llschloesser]

Is there any value in treating xi as a 5th distortion parameter? It would simplify the interface and allow one to store a pinhole, fisheye, or omni calibration model all in the same data structure (without exposing xi to models that don't care about it).

[rgolovanov]

@llschloesser I would consider this suggestion detached from this fix. Could you create separate issue about this?

[llschloesser]

@rgolovanov , thanks, yes, I will do that.