To add option for saving rendering state on disk and resume rendeing

[buddhi1980]

This is requested enhancement which will allow to pause rendering, save rendering state on disk and then resume it from disk. It is not a simple change in the code and need many changes in rendering engines.

[trinitronx]

@buddhi1980 : Is this currently possible right now to stop the GUI app render and resume later with the CLI?

For example:

1. User started a flight path render of 3600 frames in GUI app
   • Initial estimate was ~24h
   • Came back later, estimate grew to 6 days!
   • CPU is needed for other tasks during this time, so user needs ability to pause / resume the render
2. Stop the running flight path render at a certain frame
   • For our example, let's say it completed rendering up to frame 500
   • So user stops GUI app render at: frame 500
3. Computer's CPU / GPU can now used for other applications
4. Later, user resumes rendering with CLI at the next frame: 501
   • mandelbulber2 --flight --start 501 --end 3600 /path/to/settings_file_with_flight_path.fract

Note: Currently using mandelbulber2 version 2.20 on Ubuntu Linux 20.04