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