[kcmkwin/compositing] Fix effect video previews

Summary:
Currently, video previews do not work because we try to call showHide()
function that was removed in 787c39cd42.

Reviewers: #kwin, #plasma, davidedmundson

Reviewed By: #kwin, #plasma, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18720

kcmkwin/kwincompositing/qml/Effect.qml

@@ -129,9 +129,6 @@ Rectangle {
                         videoItem.visible = false;
                     }
                 }
-                onLoaded: {
-                    videoItem.item.showHide();
-                }
             }
         }
         Item {

kcmkwin/kwincompositing/qml/Video.qml

@@ -26,8 +26,7 @@ import QtMultimedia 5.0 as Multimedia
 
 Multimedia.Video {
     id: videoItem
-    autoLoad: false
-    visible: false
+    autoPlay: true
     source: model.VideoRole
     width: 400
     height: 400