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