output: use the software mixer plugin

Do all the software volume stuff inside each output thread, not in the
player thread.  This allows one software mixer per output device, and
also allows the user to configure the mixer type (hardware or
software) for each audio output.

This moves the global "mixer_type" setting into the "audio_output"
section, deprecating the "mixer_enabled" flag.

src/player_thread.c

@@ -423,8 +423,6 @@ static bool
 play_chunk(struct song *song, struct music_chunk *chunk,
 	   const struct audio_format *format, double sizeToTime)
 {
-	bool success;
-
 	assert(music_chunk_check_format(chunk, format));
 
 	if (chunk->tag != NULL) {
@@ -455,18 +453,6 @@ play_chunk(struct song *song, struct music_chunk *chunk,
 	pc.elapsed_time = chunk->times;
 	pc.bit_rate = chunk->bit_rate;
 
-	/* apply software volume */
-
-	success = pcm_volume(chunk->data, chunk->length,
-			     format, pc.software_volume);
-	if (!success) {
-		g_warning("pcm_volume() failed on %u:%u:%u",
-			  format->sample_rate, format->bits, format->channels);
-		pc.errored_song = dc.current_song;
-		pc.error = PLAYER_ERROR_AUDIO;
-		return false;
-	}
-
 	/* send the chunk to the audio outputs */
 
 	if (!audio_output_all_play(chunk)) {