decodebin3: add a fallback function when upstream changes and flush seek

When we try to flush seek to playbin3 pipeline, multiqueue pre is ongoing upstream changes with auto-plugging.

In this scenario, below symptoms are observed.

  • Seek event can't be delivered to from multiqueue pre to source part. Because a pad of this part is not linked during auto plugging.
  • flush-start/stop event could not be delivered to decoder and sink elements by pad not linked.

To avoid this symptom, we try to seek event to parsebin sink and make sure flush-start and flush-stop are delivered to decoder and sink elements.

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