qt: perform surface creation in the main thread

As is required when creating a QWindow instance as set out in the Qt documentation.

ext/qt/qtglrenderer.cc

@@ -366,7 +366,8 @@ bool GstQuickRenderer::init (GstGLContext * context, GError ** error)
             while (!m_sharedRenderData->m_surface) {
                 /* XXX: This might deadlock with the main thread if the
                  * QCoreApplication is not running and will not be able to
-                 * execute */
+                 * execute.  We only wait for 5 seconds until a better
+                 * approach can be found here */
                 if (!g_cond_wait_until (&m_sharedRenderData->cond,
                         &m_sharedRenderData->lock, end_time)) {
                     g_set_error (error, GST_RESOURCE_ERROR,