Inform Screens about size change

Summary:
Changes to a Wayland output that could affect the overall geometry
need to be communicated to the Screens class.

Test Plan: Compiles, runtime tested with other patches for rotation.

Reviewers: KWin, zzag

Reviewed By: KWin, zzag

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D25900