When the picker picks a vtkImageStack, it gets the mapper from the
currently active image on the stack.  Unfortunately, though, instead
of getting the matrix from the image, it was getting the matrix of the
stack itself (usually identity), This is now fixed, through the use
of a simple assembly path for the stack that contains only the active
image.  This also fixes the Bounds computation, which required a new
regression test image for TestImageStack.