Use threaded renderer by default in linux. This improves performances.

app/main.cpp

@@ -33,6 +33,10 @@ int main(int argc, char *argv[])
     // This disables QT appmenu under Ubuntu, which is not working with QML apps.
     setenv("QT_QPA_PLATFORMTHEME", "", 1);
 
+#if defined (Q_OS_LINUX)
+    setenv("QSG_RENDER_LOOP", "threaded", 0);
+#endif
+
 #if defined(Q_OS_MAC)
     // This allows UTF-8 characters usage in OSX.
     setenv("LC_CTYPE", "UTF-8", 1);