[Paraview] Dropping support for OpenGL1 in ParaView 5.2.

[Utkarsh Ayachit]

Folks,

As we continue to make progress on the new OpenGL2 rendering backend,
I wanted to get a feel from the community if there's any objection to
dropping support entirely for the legacy OpenGL1 rendering backend in
ParaView for 5.2?

Note this will only affect ParaView and ParaView-based applications.

Utkarsh