Export rootWindow to the QXcbNativeInterface

This change exports the root window to the native interface as there
is QX11Info::appRootWindow which so far goes over the QDesktopWidget
to get the window id of the root window. Which is a rather hackish way
considering that the root window is known to the QXcbConnection.

But even more it's a very fragile way and can result in crashes on
startup of applications if the application accesses the appRootWindow
too early.

Change-Id: Ibb09a7fa714cb355f579298fc6df33bf80f73f58
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Gatis Paeglis <gatis.paeglis@digia.com>

src/plugins/platforms/xcb/qxcbnativeinterface.cpp

@@ -80,7 +80,8 @@ static int resourceType(const QByteArray &key)
         QByteArrayLiteral("glxcontext"), QByteArrayLiteral("apptime"),
         QByteArrayLiteral("appusertime"), QByteArrayLiteral("hintstyle"),
         QByteArrayLiteral("startupid"), QByteArrayLiteral("traywindow"),
-        QByteArrayLiteral("gettimestamp"), QByteArrayLiteral("x11screen")
+        QByteArrayLiteral("gettimestamp"), QByteArrayLiteral("x11screen"),
+        QByteArrayLiteral("rootwindow")
     };
     const QByteArray *end = names + sizeof(names) / sizeof(names[0]);
     const QByteArray *result = std::find(names, end, key);
@@ -142,6 +143,9 @@ void *QXcbNativeInterface::nativeResourceForIntegration(const QByteArray &resour
     case X11Screen:
         result = x11Screen();
         break;
+    case RootWindow:
+        result = rootWindow();
+        break;
     default:
         break;
     }
@@ -264,6 +268,15 @@ void *QXcbNativeInterface::x11Screen()
     return 0;
 }
 
+void *QXcbNativeInterface::rootWindow()
+{
+    QXcbIntegration *integration = static_cast<QXcbIntegration *>(QGuiApplicationPrivate::platformIntegration());
+    QXcbConnection *defaultConnection = integration->defaultConnection();
+    if (defaultConnection)
+        return reinterpret_cast<void *>(defaultConnection->rootWindow());
+    return 0;
+}
+
 void QXcbNativeInterface::setAppTime(QScreen* screen, xcb_timestamp_t time)
 {
     static_cast<QXcbScreen *>(screen->handle())->connection()->setTime(time);

src/plugins/platforms/xcb/qxcbnativeinterface.h

@@ -70,7 +70,8 @@ public:
         StartupId,
         TrayWindow,
         GetTimestamp,
-        X11Screen
+        X11Screen,
+        RootWindow
     };
 
     QXcbNativeInterface();
@@ -94,6 +95,7 @@ public:
     void *getTimestamp(const QXcbScreen *screen);
     void *startupId();
     void *x11Screen();
+    void *rootWindow();
     static void setAppTime(QScreen *screen, xcb_timestamp_t time);
     static void setAppUserTime(QScreen *screen, xcb_timestamp_t time);
     static void *eglContextForContext(QOpenGLContext *context);