Disable smoothing thumbnails in LowResourceUsageMode

Summary:
If LowResourceUsageMode is enabled, do not waste resources on smoothing-scaling small thumbnails.

Depends on D21406

Reviewers: Gwenview, ngraham

Reviewed By: Gwenview, ngraham

Subscribers: ngraham, Gwenview

Tags: Gwenview

Differential Revision: https://phabricator.kde.org/D21407