Compatibility with VTK OpenGL2 backend

[codeling]

When compiling open_iA with OpenGL2 backend selected in VTK, there are several issues:

• There is an access violation in open with data type conversion (dlg_datatypeconversion.cpp:716, method SetupSliceWidget)
• The 3D magic lens not shown
• (potentially with specific open options only, e.g. "Open TLGI-CT data"?): 3D view has problems - no volume is shown; when trying to show multiple volumes, only first is shown; orientation marker widget crashes
• Image data (bmp, ...) loading causes crash
• The 2D magic lens shows only black
• The 2D magic lens shows no border around image in "offset" mode (just the border around the magic lens itself)

There could also be more issues that haven't been found yet.

[codeling]

Fixed in develop, will be released with 2018.11