player_thread: move cross-fading to output thread

Remove cross_fade_apply(), and call pcm_mix() in the output thread,
mixing the chunk and chunk->other together.

src/output_init.c

@@ -191,6 +191,8 @@ audio_output_init(struct audio_output *ao, const struct config_param *param,
 	ao->pause = false;
 	ao->fail_timer = NULL;
 
+	pcm_buffer_init(&ao->cross_fade_buffer);
+
 	/* set up the filter chain */
 
 	ao->filter = filter_chain_new();