@jonas I would not say that I completely gave up, but I just started a new position in a company so I lost that whole free time to mess with this particular thing
I went making https://github.com/guprobr/WakkaQt instead to learn a bit more Qt before resuming this task
The last ideas we discussed, I never tried... but the last post on projectM made me feel second toughts on the viability of the idea.... anyway, i want to resume this task someday I don't think we should give up but certainly something is missing in the backend inner parts of how Qt works with OpenGL and how projectM works with OpenGL, as it was discussed on the thread on github.
But the first thing i would do is trying with latest Qt LTS, before anything.
Second thing, to try with the same version on Windows!
We have to figure where the problem is. Obviously its a Qt issue, but what made me stop working on it was when SDL window version showed the same issues. That was very disappointing.
PS: sorry to take so long to answer, I just saw this today