[PointSet interaction] Correctly determine the closest point to a certain position

Summary:
When in editing state of the PointSet interaction view (+ toggle button at the bottom), the current point (sphere in 3-d render window) is supposed to be colored red.
The current point is the point the mouse pointer is hovering at the moment.
When two points are close together and you zoom in on them in the 3-d render window, the reaction is triggered correctly as soon as the mouse pointer hovers over a sphere.
However, the search that is triggered as reaction discontinues too early and misses out possible closer candidates, ending in coloring the wrong sphere.
This Diff fixes the search loop.

Fixes T30262.

Test Plan: See T30262.

Reviewers: floca, O1 MITK Reviewer Group I

Reviewed By: floca, O1 MITK Reviewer Group I

Maniphest Tasks: T30262

Differential Revision: https://phabricator.mitk.org/D886