Fix cell picker for images with negative spacing.

If the spacing of the image data was negative in any of the three
dimensions, it resulted in a failed bounding box test for the image
in the vtkCellPicker.  This patch adds a check for negative spacing.

Change-Id: Ia6b7c31fa78988900ce2e68f0ee8cd9bc8ecbf8d
Merge topic 'cell-picker-image-spacing' into master

8eb00d2f Fix cell picker for images with negative spacing.