Summary:
Let the data saved in OutputDeviceInterface be the single source of truth
and as low hanging fruits first do this for global position and scale.

Test Plan: Nested Wayland, Drm, virtual backends tested.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Maniphest Tasks: T11459

Differential Revision: https://phabricator.kde.org/D23489