16130: Fix inconsistency in vtkImageSliceCollection.
The Bottom member of vtkCollection was not updated if the added slice eplaced the previous bottom slice of the collection. This caused an inconsistency in the data structure which could lead to a crash in subsequent operations. I have rewritten the insertion code to reduce its complexity.
Original bug report: http://www.vtk.org/Bug/view.php?id=16130
This bug is present in VTK 5.10 through to VTK 7.0, inclusive.