Support kwin fractional scaling in wayland backend

Summary:
Note KScreen UI is deliberately not updated as full functionality is not
complete. However, I think it's fine to merge this and have it exposed via
only kscreen-doctor for devs.

Test Plan:
Used kscreen-doctor with modified kwin
Kwin got float values

Reviewers: KWin, mart

Reviewed By: KWin, mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D13617