[FrameSvgItem] Update filtering on smooth changes

Summary:
When just the smooth property is changed, this also results in a call of
QQuickItem::updatePaintNode reimplementations. So always just set the
filtering flag to match the current value of the smooth property.

Reviewers: Plasma, broulik, mart

Reviewed By: Plasma, broulik

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D20579