Comment # 7 on bug 79756 from Michel Dänzer

(In reply to comment #6)
> Yeah, still crashes with Glamor as of git 081a537.

That's the standalone glamor tree. I mean glamor from the current xserver Git
tree.

Anyway, I think the attached backtrace can only happen if Steam uses GLX
indirect rendering (current versions of Steam present a warning dialog on
startup when that's the case). You can try starting Steam with
LIBGL_DEBUG=verbose to see why it's not getting direct rendering.