audio: don't downmix when doing digital passthrough

This obviously doesn't work. It wasn't much of a problem in the past
because most passthrough formats use 2 channels, which is also the
default for downmix.

player/audio.c

@@ -143,6 +143,7 @@ void reinit_audio_chain(struct MPContext *mpctx)
         ao_format   = out_format.format;
         ao_channels = out_format.channels;
     } else {
+        if (!AF_FORMAT_IS_SPECIAL(in_format.format))
             ao_channels = opts->audio_output_channels; // automatic downmix
     }