[SDDM theme] Use the biggest wallpaper resolution

Summary: Starting from Plasma 5.16 we have the Next wallpaper is available in 5K resolution so we can make use of it in the login screen as well.

Reviewers: VDG, Plasma, ngraham

Reviewed By: VDG, ngraham

Subscribers: davidedmundson, ngraham, plasma-devel

Tags: Plasma

Maniphest Tasks: T10269

Differential Revision: https://phabricator.kde.org/D18007