This is my branch:
Shading with multi-volume rendering works with this branch, however, not every time! Unfortunately about 2/3 of the time there is no shading at all (looks black, as if lighting is not considered whatsoever), but when it shows up, then it looks as it’s supposed to. I haven’t been able to figure out the reason for this undeterministic behavior. Any suggestions are welcome, and I’m happy to answer any questions.