Get output physical size from output device
Get the physical size directly from the always available output device
interface instead of saving an additional copy in the abstract wayland
There is some ambiguity with orientation and naming that needs to be
cleaned up when output orientation is reworked.
Test Plan: Nested Wayland, Drm, virtual backends tested.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Maniphest Tasks: T11459
Differential Revision: https://phabricator.kde.org/D23496