output: pass audio_format to plugin.init() and plugin.open()

Pass the globally configured audio_format as a const pointer to
plugin.init().  plugin.open() gets a writable pointer which contains
the audio_format requested by the plugin.  Its initial value is either
the configured audio_format or the input file's audio_format.

src/output_thread.c

@@ -75,7 +75,7 @@ static void *audio_output_task(void *arg)
 
 		case AO_COMMAND_OPEN:
 			assert(!ao->open);
-			ao->plugin->open(ao);
+			ao->plugin->open(ao, &ao->outAudioFormat);
 			ao_command_finished(ao);
 			break;