videoio(gstreamer): support dumping of VideoWriter pipeline

modules/videoio/src/cap_gstreamer.cpp

@@ -1657,6 +1657,8 @@ bool CvVideoWriter_GStreamer::open( const char * filename, int fourcc,
     }
 #endif
 
+    GST_DEBUG_BIN_TO_DOT_FILE(GST_BIN(pipeline), GST_DEBUG_GRAPH_SHOW_ALL, "write-pipeline");
+
     stateret = gst_element_set_state(GST_ELEMENT(pipeline), GST_STATE_PLAYING);
     if(stateret  == GST_STATE_CHANGE_FAILURE) {
         handleMessage(pipeline);