Perform detection of CompositingPrefs async

Detecting CompositingPrefs invokes an external program. Waiting
for this can be moved in a second thread.

Due to the introduction of the thread the initialization order
of KWin is changed: the WindowManager is initialized before the
Compositor. Interestingly this makes KWin felt more responsive
as the screen is not frozen for several seconds.

REVIEW: 104579

composite.cpp

@@ -57,6 +57,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 #include <stdio.h>
 
+#include <QtCore/QtConcurrentRun>
+#include <QtCore/QFutureWatcher>
 #include <QMenu>
 #include <QTimerEvent>
 #include <kaction.h>
@@ -90,9 +92,27 @@ void Workspace::setupCompositing()
         return;
     }
 
-    if (!options->isCompositingInitialized())
+    if (!options->isCompositingInitialized()) {
+#ifndef KWIN_HAVE_OPENGLES
+        // options->reloadCompositingSettings(true) initializes the CompositingPrefs which calls an
+        // external program in turn
+        // run this in an external thread to make startup faster.
+        QFutureWatcher<void> *compositingPrefsFuture = new QFutureWatcher<void>();
+        connect(compositingPrefsFuture, SIGNAL(finished()), this, SLOT(slotCompositingOptionsInitialized()));
+        connect(compositingPrefsFuture, SIGNAL(finished()), compositingPrefsFuture, SLOT(deleteLater()));
+        compositingPrefsFuture->setFuture(QtConcurrent::run(options, &Options::reloadCompositingSettings, true));
+#else
+        // OpenGL ES does not call the external program, so no need to create a thread
         options->reloadCompositingSettings(true);
+        slotCompositingOptionsInitialized();
+#endif
+    } else {
+        slotCompositingOptionsInitialized();
+    }
+}
 
+void Workspace::slotCompositingOptionsInitialized()
+{
     char selection_name[ 100 ];
     sprintf(selection_name, "_NET_WM_CM_S%d", DefaultScreen(display()));
     cm_selection = new KSelectionOwner(selection_name);

workspace.h

@@ -645,6 +645,10 @@ private slots:
     void slotBlockShortcuts(int data);
     void slotReloadConfig();
     void setupCompositing();
+    /**
+     * Called from setupCompositing() when the CompositingPrefs are ready.
+     **/
+    void slotCompositingOptionsInitialized();
     void finishCompositing();
     void fallbackToXRenderCompositing();
     void performCompositing();