Thank’s rdb, but I’ve already specified it as a comment in my code above 
For now I prefer to force p3tinydisplay to be sure that everything I want to do is rendered correctly using this API, and later I will use aux-display.
Follow-up on my question: I would now like to fetch the rendered image directly to a Python variable (a matrix containing the intensity of every pixel).
I can already do that by saving the rendered image on disk and then load it using Python, but I would prefer to economize that additional cost on drive by fetching the rendered image directly from RAM.
I’ve found the method extractTextureData(), I’ll try to use that, but I’m not sure it will work out as expected, so if anyone has got a better idea on the subject…