Add variant opengl2, which is ON by default
Add variant opengl2, which is ON by default

Needed for visit which depends on vtk@6.1.0~opengl2

EmreAtes/spack,tmerrick1/spack,lgarren/spack,LLNL/spack,LLNL/spack,iulian787/spack,TheTimmy/spack,matthiasdiener/spack,matthiasdiener/spack,EmreAtes/spack,tmerrick1/spack,iulian787/spack,iulian787/spack,tmerrick1/spack,krafczyk/spack,TheTimmy/spack,LLNL/spack,skosukhin/spack,krafczyk/spack,krafczyk/spack,skosukhin/spack,mfherbst/spack,EmreAtes/spack,iulian787/spack,mfherbst/spack,lgarren/spack,iulian787/spack,krafczyk/spack,EmreAtes/spack,mfherbst/spack,LLNL/spack,matthiasdiener/spack,matthiasdiener/spack,TheTimmy/spack,lgarren/spack,tmerrick1/spack,TheTimmy/spack,mfherbst/spack,mfherbst/spack,lgarren/spack,lgarren/spack,TheTimmy/spack,skosukhin/spack,tmerrick1/spack,matthiasdiener/spack,krafczyk/spack,skosukhin/spack,LLNL/spack,skosukhin/spack,EmreAtes/spack