MRView tractography tool: remove visibility tests in shaders
MRView tractography tool: remove visibility tests in shaders

As discussed in #281 - this made no difference on performance on my
system.

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