[sddm-theme/lock screen] Adjust login and lock screen blur effect

Summary: This patch modifies the login screen and lock screen blur effect.

Test Plan:
Before:
{F6643726}
After:
{F6643728}

Any and all input is appreciated!

Reviewers: #vdg, ngraham, #plasma, davidedmundson, filipf

Reviewed By: #vdg, ngraham

Subscribers: plasma-devel

Tags: #plasma

Maniphest Tasks: T10325

Differential Revision: https://phabricator.kde.org/D19409