The volume is not updated properly when removing the required initial contours (in 2D and 3D view).

you can reproduce the problem by conducting the following steps:

1. load Pic3D.nrrd
2. open segmentation plugin - enable 3D interpolation
3. draw contour in render window
4. draw second contour in different slice -- a volume should appear
5. subtract one of the contours (or both)

result: the volume does not update (e.g. it does not disappear)

6. draw a new contour

result: the old volume is considered in calculation

desired behaviour: The volume should disappear as soon as there is only a single contour active in the render window.