Where are framebuffers/visuals created?

[Bengt Ahlgren]

I'd like to try to debug and find the cause of this issue with Kwin, but
I don't know where in the graphics stack to start digging:

https://mail.kde.org/pipermail/kde-freebsd/2018-April/027704.html

In short, after an update of Mesa, Kwin complained about not finding the
appropriate framebuffer, and refused doing compositing with opengl.

So, my question is: where in the graphics stack are the
framebuffers/visuals created?

Bengt