Merge remote-tracking branch 'cus/stable'

* cus/stable:
  ffplay: add support for changing the channel by the C key
  ffplay: cycle through the streams of the current program, and not every stream
  ffplay: add null packet after attached pics packet
  ffplay: factor out putting null packet into the queue

Merged-by: Michael Niedermayer <michaelni@gmx.at>