pleorasrc: add packet-size property

Also, if auto-negotiate (packet-size=0) is used (the default), and
negotiation fails, use the safe packet size of 1476 bytes.

sys/pleora/gstpleorasrc.h

@@ -55,6 +55,7 @@ struct _GstPleoraSrc
   gchar *multicast_group;
   gint port;
   gboolean receiver_only;
+  gint packet_size;
 
   guint32 last_frame_count;
   guint32 total_dropped_frames;