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/crossfade.c

@@ -131,46 +131,3 @@ unsigned cross_fade_calc(float duration, float total_time,
 
 	return chunks;
 }
-
-void cross_fade_apply(struct music_chunk *a, const struct music_chunk *b,
-		      const struct audio_format *format,
-		      float mix_ratio)
-{
-	size_t size;
-
-	assert(a != NULL);
-	assert(b != NULL);
-	assert(a->length == 0 || b->length == 0 ||
-	       audio_format_equals(&a->audio_format, &b->audio_format));
-
-	if (a->tag == NULL && b->tag != NULL)
-		/* merge the tag into the destination chunk */
-		a->tag = tag_dup(b->tag);
-
-	size = b->length > a->length
-		? a->length
-		: b->length;
-
-	pcm_mix(a->data,
-		b->data,
-		size,
-		format,
-		mix_ratio);
-
-	if (b->length > a->length) {
-		/* the second buffer is larger than the first one:
-		   there is unmixed rest at the end.  Copy it over.
-		   The output buffer API guarantees that there is
-		   enough room in a->data. */
-
-#ifndef NDEBUG
-		if (a->length == 0)
-			a->audio_format = b->audio_format;
-#endif
-
-		memcpy(a->data + a->length,
-		       b->data + a->length,
-		       b->length - a->length);
-		a->length = b->length;
-	}
-}