Create correct OSMesa context.
Create correct OSMesa context.

Bringing in changes from vtkOSOpenGLRenderWindow to
vtkXOpenGLRenderWindow to create correct OSMesa context for OpenGL2
backend.

keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix