ffplay: update and extend documentation for channel and stream switching

Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: factor out picture freeing code

Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: use av_frame_get_pkt_pos instead directly accessing pkt pos

Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: add support for libswresample options

Signed-off-by: Marton Balint <cus@passwd.hu>
Merge remote-tracking branch 'cus/stable'

* cus/stable:
  ffplay: add support for libswresample options
  ffplay: use av_frame_get_pkt_pos instead directly accessing pkt pos
  ffplay: factor out picture freeing code
  ffplay: update and extend documentation for channel and stream switching

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