Don't unintentionally change font rendering when rendering preview images

Summary:
Font rendering settings are changed temporarily to render preview images. The changed settings were
reverted, but never reapplied. This way rendering preview images could alter the user's font rendering
configuration.

Reviewers: ngraham

Reviewed By: ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14480