Expand support for the OpenGL2 backend

This change makes more modules dynamically switch between the
rendering backends if they have no OpenGL code, or introduces some
ifdefs in the case of RenderingParallel (which may not be functional
at this stage). Added RenderingFreeTypeOpenGL2 which will also need
some additional work before it is properly integrated with the new
rendering backend, but it surprisingly seems to render on the
desktop.

This commit contains everything needed to get a minimal ParaView
build using the new rendering backend for polygonal rendering.

Change-Id: If8676d41d263b046a54060965fc760b5d4bd64a1
Merge topic 'opengl2-backend' into master

666ade8a Expand support for the OpenGL2 backend