Use raster graphics on X11 by default

This will be the default in Qt with 4.8. It is faster and
less prone to graphics issues than the X11 engine so we
should use it in Creator by default.

task-number: QTCREATORBUG-2238

src/app/main.cpp

@@ -183,6 +183,9 @@ int main(int argc, char **argv)
     setrlimit(RLIMIT_NOFILE, &rl);
 #endif
 
+#ifdef Q_WS_X11
+    SharedTools::QtSingleApplication::setGraphicsSystem("raster");
+#endif
     SharedTools::QtSingleApplication app((QLatin1String(appNameC)), argc, argv);
 
     const int threadCount = QThreadPool::globalInstance()->maxThreadCount();