Ok I can reproduce the flickering here, when  removing the ```prio=1```.
If you add a print of the prio instead you'll get funny numbers. (garbage essentially), but we'd expect to see only zeros then, as the Renderables prio was set to zero in the constructor. 

* we need to understand where the garbage comes from and eliminate that as the source for the flickering.

* If not succeeding at finding the garbage generator, I'd rather set prio to 0 instead of 1 here.