Fix incorrect offset passing to sliding popups effect

For the generic case without an explicit specified offset, the
offset calculation was incorrect. Instead of calculating the offset
we let kwin decide and indicate it by an offset of -1.
REVIEW: 103366

windoweffects.cpp

@@ -134,22 +134,19 @@ void slideWindow(QWidget *widget, Plasma::Location location)
     Display *dpy = QX11Info::display();
     Atom atom = XInternAtom( dpy, "_KDE_SLIDE", False );
     QVarLengthArray<long, 2> data(2);
+    data[0] = -1;
 
     switch (location) {
     case LeftEdge:
-        data[0] = widget->geometry().left();
         data[1] = 0;
         break;
     case TopEdge:
-        data[0] = widget->geometry().top();
         data[1] = 1;
         break;
     case RightEdge:
-        data[0] = widget->geometry().right();
         data[1] = 2;
         break;
     case BottomEdge:
-        data[0] = widget->geometry().bottom();
         data[1] = 3;
     default:
         break;