Use VTKs OpenGL2 backend - use VTK 8.0.1 or 6.1.0 on Mac.

[MattClarkson]

I want to test this out specifically for Mac.

1. According to https://blog.kitware.com/new-opengl-rendering-in-vtk/, this was merged on June 20, 2014, so we would need at least VTK version 6.3. In VTK 6.3, both OpenGL and OpenGL2 backends should be present.
2. At UCL, Im currently using Qt 5.4.2 for MITK based projects, using a fork of MITK 2015.5.2. But Qt changed their OpenGL significantly in 5.5, and MITK/master now has a minimum Qt 5.6. So, the target is Qt 5.6 or newer.
3. When I first compiled this project with Qt 5.6, I got issues on Mac, similar to: opencv does not build on Mac OS X 10.10 with Xcode 7 opencv/opencv#7606, and further googling suggests we may need to specify a minimum Mac SDK or xtools version.
4. Then, Im also aware that on a Mac, there was this issue pcl_visualization build failure on Mac OS X PointCloudLibrary/pcl#712 when I was integrating PCL, whereby VTK 7.1 would not build, so I dropped back to 6.1. So the first question is whether VTK 6.3 will compile at all, regardless of the backend. This commit Kitware/VTK@a3e9fc9 suggests it should not. But then maybe a newer version of PCL will work?

[MattClarkson]

Tested on Windows, Linux and Mac, but with OpenCV off. If OpenCV doesn't compile with a specific version, then that should be a separate ticket. This is about VTK / Qt and PCL versions.