Fix vtk compiler errors

[sandro-elsweijer]

Describe your changes here:
The main branch is not compilable with vtk

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[jmark]

I can compile the main branch with enabled vtk without any issues. I followed the instructions in the wiki: https://github.com/DLR-AMR/t8code/wiki/Installation-of-VTK.

Or are you compiling with pedantic and similar?

[sandro-elsweijer]

I can compile the main branch with enabled vtk without any issues. I followed the instructions in the wiki: https://github.com/DLR-AMR/t8code/wiki/Installation-of-VTK.

Or are you compiling with pedantic and similar?

This is only an Issue with VTK < 9.2. This can be observed here: https://github.com/DLR-AMR/t8code/actions/runs/4973174875/jobs/8898841856
This also fixes the compiler warnings caused by uninitialized variables