decodebin3: clear the is_drained flag of slot if receive flush-stop event
requested to merge elliot_chen/gstreamer:decodebin3-do-not-remove-elements-if-receive-EOS-from-all-tracks into main
- Some demuxers will send EOS directly for audio track if play rate is greater than 2 or less than 0.
- In this case, just audio slot receive EOS and the is_drained flag of the slot can't be clear.
- It will cause hang if playback do new trick mode operation.