[Paraview] Removing legacy rendering backend from ParaView 5.5

[Utkarsh Ayachit]

Folks,

Final ParaView 5.4 should be tagged and released in the next few days.
This may be a good time to discuss dropping support for legacy
components.

I'd like to propose dropping support for legacy rendering backend in
lieu of the what we call the OpenGL2 rendering backend that requires
OpenGL 3.2 or newer. We have been using the newer backend by default
since 5.0 (January 2016) and is indeed the most tested and used
rendering backend since then.

If there are strong reasons to continue supporting the legacy backend
beyond 5.4 please make us aware before June 30, 2017. One can always
use older versions of ParaView, if required. For the most part, we
have already stopped doing any development, including maintenance
fixes for the legacy rendering backend.

Utkarsh