Fix multithreaded GL rendering

Call XInitThread() before any other call to Xlib.

Change-Id: I88d3a87bf31dab06b0a4c6a4cd1d32979cd70d4a
Reviewed-on: http://codereview.qt.nokia.com/3571
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Jørgen Lind <jorgen.lind@nokia.com>

src/plugins/platforms/xcb/qxcbintegration.cpp

@@ -81,6 +81,10 @@ QXcbIntegration::QXcbIntegration(const QStringList &parameters)
 {
     QGuiApplicationPrivate::instance()->setEventDispatcher(m_eventDispatcher);
 
+#ifdef XCB_USE_XLIB
+    XInitThreads();
+#endif
+
     m_connections << new QXcbConnection;
 
     for (int i = 0; i < parameters.size() - 1; i += 2) {