Separating ITKVtkGlue into "image conversion" and "rendering" bridges

[romangrothausmann]

Description

As discussed in
https://discourse.vtk.org/t/module-vtkrenderingnone-not-built-even-with-vtk-rendering-backend-none/585
ITKVtkGlue currently depends on vtkRendering* with a back-end other than None.
As pointed out by @dzenanz: ITKVtkGlue has an image conversion class and a rendering class, which are "in need of maintenance".

Impact analysis

The current design bridges ITK with VTK but does not allow to choose only image conversion or rendering.
This becomes critical when designing docker images where the need for an OpenGL implementation is an extra burden especially if that is only needed to satisfy a cmake check and then never used, see e.g. romangrothausmann/ITK-VTK_CLIs@d2ba704 and log

Expected behavior

Ideally, it would be great to be able to choose interaction with VTK concerning image conversion separately from rendering.

Actual behavior

ITKVtkGlue enables both image conversion and rendering.

Versions

Tested with ITK-4.12.2/ITK-4.13.2 and VTK-7.1.1, VTK-7 because
VTK_RENDERING_BACKEND is gone since VTK-8 (see #424) and therefore this is the latest combination of ITK and VTK that were expected to work.

Environment

Linux ubuntu:16.04, see https://github.com/romangrothausmann/ITK-VTK_CLIs/blob/b4dfe7d68dd7022e66d5d1ea686715334631ef46/Dockerfile