Merge remote-tracking branch 'cus/stable'

* cus/stable:
  ffplay: do not cycle through unavailable show modes
  ffplay: add option to disable subtitling
  ffplay: use NAN to signal invalid external clock
  ffplay: if audio or video clock is invalid return NAN
  ffplay: allow frame dropping if we redisplay an already displayed frame
  ffplay: return true for pictq_prev_picture if it was successful
  ffplay: only quit from audio_decode_frame before decoding when paused
  ffplay: drop remaining frames in current audio avpacket when seeking
  ffplay: signal seek event to read thread
Merged-by: Michael Niedermayer <michaelni@gmx.at>