backport Qt 5.7 change to 5.6.1

Add QX11Info::isCompositingManagerRunning() function

Task-number: QTBUG-41195

src/x11extras/qx11info_x11.cpp

@@ -370,4 +370,29 @@ xcb_connection_t *QX11Info::connection()
     return reinterpret_cast<xcb_connection_t *>(connection);
 }
 
+/*!
+    \since 5.7
+
+    Returns true if there is a compositing manager running for the connection
+    attached to \a screen.
+
+    If \l -1, the application primary screen is used.
+*/
+bool QX11Info::isCompositingManagerRunning(int screen)
+{
+    if (!qApp)
+        return false;
+    QPlatformNativeInterface *native = qApp->platformNativeInterface();
+    if (!native)
+        return false;
+
+    QScreen *scr = screen == -1 ?  QGuiApplication::primaryScreen() : findScreenForVirtualDesktop(screen);
+    if (!scr) {
+        qWarning() << "isCompositingManagerRunning: Could not find screen number" << screen;
+        return false;
+    }
+
+    return native->nativeResourceForScreen(QByteArray("compositingEnabled"), scr);
+}
+
 QT_END_NAMESPACE

src/x11extras/qx11info_x11.h

@@ -67,6 +67,8 @@ class Q_X11EXTRAS_EXPORT QX11Info
     static Display *display();
     static xcb_connection_t *connection();
 
+    static bool isCompositingManagerRunning(int screen = -1);
+
 private:
     QX11Info();
 };

tests/auto/qx11info/tst_qx11info.cpp

@@ -101,6 +101,8 @@ void tst_QX11Info::staticFunctionsBeforeQApplication()
     appUserTime = QX11Info::appUserTime();
     QCOMPARE(appTime, 0ul);
     QCOMPARE(appTime, 0ul);
+
+    QX11Info::isCompositingManagerRunning();
 }
 
 static const char idFromEnv[] = "startupid_TIME123456";