dashdemux: Make sure to pass the next buffer after a seek with DISCONT flag

When we manually seek for skipping ahead in keyunit-only trickmode, we
have to enforce that ourselves as adaptivedemux does not know about the
seeks.

ext/dash/gstdashdemux.c

@@ -1537,6 +1537,8 @@ gst_dash_demux_stream_seek (GstAdaptiveDemuxStream * stream, gboolean forward,
     }
   }
 
+  stream->discont = TRUE;
+
   return GST_FLOW_OK;
 }