Made QWidget::setWindowOpacity work with the XCB platform plugin.

Ported the rather trivial implementation from Qt 4.

Task-number: QTBUG-28477
Change-Id: I44e14a3c150af43c6b9b880242b655fe06084fd7
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

src/plugins/platforms/xcb/qxcbwindow.cpp

@@ -1786,6 +1786,23 @@ static inline xcb_rectangle_t qRectToXCBRectangle(const QRect &r)
     return result;
 }
 
+void QXcbWindow::setOpacity(qreal level)
+{
+    if (!m_window)
+        return;
+
+    quint32 value = qRound64(qBound(qreal(0), level, qreal(1)) * 0xffffffff);
+
+    Q_XCB_CALL(xcb_change_property(xcb_connection(),
+                                   XCB_PROP_MODE_REPLACE,
+                                   m_window,
+                                   atom(QXcbAtom::_NET_WM_WINDOW_OPACITY),
+                                   XCB_ATOM_CARDINAL,
+                                   32,
+                                   1,
+                                   (uchar *)&value));
+}
+
 void QXcbWindow::setMask(const QRegion &region)
 {
     if (!connection()->hasXShape())

src/plugins/platforms/xcb/qxcbwindow.h

@@ -109,6 +109,8 @@ public:
 
     bool startSystemResize(const QPoint &pos, Qt::Corner corner);
 
+    void setOpacity(qreal level);
+
 #if !defined(QT_NO_SHAPE)
     void setMask(const QRegion &region);
 #endif // !QT_NO_SHAPE