OpenGL: Prevent uninitialised variable memory warnings
OpenGL: Prevent uninitialised variable memory warnings

GLenum MR::GUI::GL::Texture::tex_type was added in 3afcd2c5, but not appropriately initialised on construction, leading to unwanted warnings when running mrview in valgrind.

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