[2pt] Implement non-real-time server-side rendering

[ivan-cukic]

[RalphSteinhagen]

linked and similar to PNG screenshot export mentioned in: fair-acc/opendigitizer#9

The idea would be to render and capture the underlying OpenGL/Vulcan frame buffer and write this to file.

Also, there are ImGUI software-only backends when used w/o a graphics card.