Get output pixel size from output device

Summary:
Get the pixel size directly from the always available output device
interface instead of saving an additional copy in the backends.

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

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Maniphest Tasks: T11459

Differential Revision: https://phabricator.kde.org/D23495