(In reply to Markus Elfring from comment #7)
> Created attachment 704421 [details]
> Logging from the X server for a fresh login
> 
> elfring@Sonne:~> glxinfo
> name of display: :0.0
> display: :0  screen: 0
> direct rendering: Yes
> server glx vendor string: NVIDIA Corporation
> server glx version string: 1.4
> ���

Ok, both look right.

Does it work if you allow to run Xorg as root?
sudo chmod +s /usr/bin/Xorg

And/or try to add the user sddm to the "video" group:
sudo usermod -a -G video sddm

The sddm greeter runs as unprivileged user sddm, but needs to have access to
the hardware. Without nvidia, this should be taken care of by the system (if
not, it is a bug ;-) ).

> How much will evolution matter also for the Qt software (like classes around
> ���QSurfaceFormat���)?

Well, if you want to discuss the evolution of the "Qt software", please do so
at the corresponding mailinglists with the actual developers:
http://lists.qt-project.org/mailman/listinfo