ffplay: implement separete audio decoder thread

Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: fix indentation after last commit

Signed-off-by: Marton Balint <cus@passwd.hu>
ffplay: only output null packet once on EOF

The generic decoder properly flushes the codecs with one null packet as well.

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