Auto tracking stop deletes already tracked point

[Morgan Turner]

When auto-tracking over a previously tracked marker, if the point is not able to be auto tracked anymore (thus, auto-tracking stops), the point data of the frame it stops on gets deleted.

Specific circumstance: tracking a particularly hard to follow sequence of frames on one camera, then using auto tracking where possible to track the point on the other camera. When auto-track can't follow anymore, the point I previously had tracked (by hand) on the first camera gets deleted for a single frame.

[Benjamin Knorlein]

fixes #61: Points gets not deleted when set manually and tracking fails

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