Audio playback improvement


Hi, popcormmix,

As reported long long time ago( #28 ), a problem about audio channel change during playback.

Some container format doesn't care about audio channels, so we should check decoded(codec output) audio format.(As ffmpeg do) And this audio format change is minor change, so we don't need close -> open codec.

I know this patch is very rough and need some improvement.

May I ask you reviewing this patch?