If the CT gantry is tilted, produce a shear matrix.

The PatientMatrix should reflect the true geometry of the volume.
So if the volume is trapezoidal, the PatientMatrix should have shear.
Add some test code that reverses a gantry tilt.
Add --stack option to TestDICOMDisplay for enh files.

Enhnaced DICOM files can have multiple stacks of images, and the
vtkDICOMReader has a SetStackID() method for choosing which stack
to load.  Therefore, it makes sense for this to also be an option
for the display example.