BUG : Update GL2PS writer to no compress HD images

There are bug when compressed in .gz when trying to extract them after
BUG : put back VTK_USE_GL2PS and add VTK_Group_Qt for VTK6

- We accidentaly removed the GL2PS when updating the external_VTK.cmake
- We put VTK_Group_Qt on ON for VTKv6, since there is no more VTK_USE_QT that we had in VTKv5