With software rendering, use outlines instead of shadows on login & lock screen

Summary:
QML shadows aren't supported with software rendering, so the white text often has poor readability.

This patch falls back to outlines when using software rendering to ensure adequate readability.

BUG: 398317
FIXED-IN: 5.15.0

Test Plan:
No visual change with hardware rendering of some sort.

When forcing software rendering, I get outlines instead of shadows

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18733