Ensure the backend has had its geometry updated.

When trying to play video in an overlay window, the video fails to
be displayed because everything that would cause updateGeometry to be
called on the backend has already occurred prior to the backend having
been created.

Change-Id: Ida3c8feea69b497a18e9f44d7143bed03380d6ae
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>

src/qtmultimediaquicktools/qdeclarativevideooutput.cpp

@@ -247,7 +247,10 @@ bool QDeclarativeVideoOutput::createBackend(QMediaService *service)
     if (!backendAvailable) {
         qWarning() << Q_FUNC_INFO << "Media service has neither renderer nor window control available.";
         m_backend.reset();
+    } else if (!m_geometryDirty) {
+        m_backend->updateGeometry();
     }
+
     return backendAvailable;
 }