[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

effects/fadedesktop/package/contents/code/main.js

@@ -54,7 +54,7 @@ effects['desktopChanged(int,int)'].connect(function(oldDesktop, newDesktop) {
                 duration: duration,
                 animations: [{
                     type: Effect.Opacity,
-                    to: w.opacity,
+                    to: 1.0,
                     from: 0.0
                 }]
             });