Fix nifti sform, qform when qfac=-1.
When the "QFac" field in the nifti header (which indicates slice ordering) was equal to -1, the vtkNIFTIImageReader would incorrectly set the offset in the SFormMatrix. And calling SetQFac(-1) on the vtkNIFTIImageWriter caused incorrect offsets to be stored in both the qform and sform of the written file. Thanks to Matthew Brown for reporting this error in the code.