[KCM GridDelegate] Use layer effect only on OpenGL backend

Authored by broulik on Aug 7 2018, 1:03 PM.

Description

[KCM GridDelegate] Use layer effect only on OpenGL backend

QtGraphicalEffects only works then, otherwise we render no thumbnail whatsoever

CCBUG: 397220

Differential Revision: https://phabricator.kde.org/D14668