Segmentation systems should use instanced drawing where they can

[Zach Pearson]

Currently the previews of a segmentation in 2D use copies of the cursor where the center of a new copy is wherever the mouse was while the previous frame was being generated.

This is very slow.

For 3D segmentations it's even slower: On every mouseMoved event the points inside the sphere used to control the 3D segmentation are filled in in the underlying volume. The slowest part of doing it live is recomputing the surface every frame, actually filling the data in is very fast. It would be good to just show a preview of what will be segmented by instancing the spheres -- maybe at a low opacity -- and then filling them in then updating the surface once.

[Zach Pearson]

Made the 3D segmentation use step 2 while segmenting to make it fast enough to feel interactive. The instanced drawing is a good thing to aim for but not necessary at this time. Slowdowns occur after five or six seconds of segmenting but most people will click and release or segment for a few seconds at a time over a small area.