chunk: added attribute "other"

Preparation for cross-fading support in the output thread.

src/buffer.c

@@ -118,6 +118,9 @@ music_buffer_return(struct music_buffer *buffer, struct music_chunk *chunk)
 	assert(buffer != NULL);
 	assert(chunk != NULL);
 
+	if (chunk->other != NULL)
+		music_buffer_return(buffer, chunk->other);
+
 	g_mutex_lock(buffer->mutex);
 
 	music_chunk_free(chunk);

src/chunk.c

@@ -27,6 +27,7 @@
 void
 music_chunk_init(struct music_chunk *chunk)
 {
+	chunk->other = NULL;
 	chunk->length = 0;
 	chunk->tag = NULL;
 	chunk->replay_gain_serial = 0;

src/chunk.h

@@ -44,6 +44,12 @@ struct music_chunk {
 	/** the next chunk in a linked list */
 	struct music_chunk *next;
 
+	/**
+	 * An optional chunk which should be mixed into this chunk.
+	 * This is used for cross-fading.
+	 */
+	struct music_chunk *other;
+
 	/** number of bytes stored in this chunk */
 	uint16_t length;