Merge remote-tracking branch 'cus/stable'

* cus/stable:
  ffplay: if there is no audio stream, use external clock by default
  ffplay: use get_master_sync_type where necessary
  ffplay: add get_master_sync_type function
  ffplay: only check external clock if current frame serial matches the displayed frame serial
  ffplay: add serial field to PacketQueue entry and populate it
  ffplay: fix external time sync mode
  ffplay: initialize audio and video pts drift
  ffplay: fix nosync threshold check in synchronize_audio
  ffplay: use framedrop by default when sync is not done to video
Merged-by: Michael Niedermayer <michaelni@gmx.at>