ffplay: factor out putting null packet into the queue

Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: add null packet after attached pics packet

Fixes ticket #2904.

Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: cycle through the streams of the current program, and not every stream

When changing the audio, video or subtitle stream, from now on, ffplay will
cycle through the streams of the current program.

Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: add support for changing the channel by the C key

Signed-off-by: Marton Balint <cus@passwd.hu>
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>