mrview: Fix 3D textures for images with non-standard header offset/scaling
mrview: Fix 3D textures for images with non-standard header offset/scaling

Closes #368.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3