Using an offscreen renderer influences completely separate interactive renderer

[ben.boeckel (Ben Boeckel (Kitware))]

@rdbrn

The offscreen renderer is executed in a separate python thread.

Hmm…how well does VTK like doing rendering from multiple threads? Are you reusing the pipeline and mappers perhaps? I wonder if OpenGL contexts get confused… (just musing out loud here)

Cc: @spyridon97 @jaswantp