Have segmentations interpolate mouse movements

[Zach Pearson]

I was experimenting with 3D slicer and moving my mouse quickly -- much too quickly for every MouseMove event to be within the same circle. I can only conclude that the reason nice straight lines of circles are displayed between any two mouse movements are that the movements are interpolated. Perhaps we should interpolate movement as well. It would certainly squash the bug where segmenting hitches for a second after the first MouseDown event and comes to life after the second one.