matroskademux: fix stream synchronization

.. by initializing streams starting at 0, as that is basically
where we 'seek to' at the start and assume streams to start elsewhere.
Also enables newsegment update events for subtitle streams.

gst/matroska/matroska-demux.c

@@ -1075,7 +1075,7 @@ gst_matroska_demux_add_stream (GstMatroskaDemux * demux)
   context->index_writer_id = -1;
   context->type = 0;            /* no type yet */
   context->default_duration = 0;
-  context->pos = GST_CLOCK_TIME_NONE;
+  context->pos = 0;
   context->set_discont = TRUE;
   context->timecodescale = 1.0;
   context->flags =