[sddm-theme/lock screen] Render shadows before (and below) labels

Test Plan:
Before: {F6640823}
After: {F6640826}

Reviewers: #vdg, ngraham, filipf

Reviewed By: #vdg, ngraham

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T10325

Differential Revision: https://phabricator.kde.org/D19366