desktop-entry-spec: add PrefersDiscreteGPU entry

[Bastien Nocera]

Hey,

I've updated Jan's patch to add a PrefersDiscreteGPU key to the
desktop-entry-spec:
https://gitlab.freedesktop.org/xdg/xdg-specs/merge_requests/13

It's a standardisation of KDE's X-KDE-RunOnDiscreteGpu.

Comments on the MR would be appreciated.

As for the implementation, I'm currently wondering how to implement it
in a way that requires minimal changes, and without bringing in some
ugly code into core libraries like GLib or Qt. See:
https://gitlab.gnome.org/GNOME/gnome-shell/issues/1804#note_631724

Cheers