[effects/fadedesktop] Fade to the correct opacity

Summary:
An opacity animation calls multiplyOpacity not setOpacity, therefore we
want to always fade between 0 and 1, not up to the window opacity.

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15476