pipe: added music_buffer, rewrite music_pipe

Turn the music_pipe into a simple music_chunk queue.  The music_chunk
allocation code is moved to music_buffer, and is now managed with a
linked list instead of a ring buffer.  Two separate music_pipe objects
are used by the decoder for the "current" and the "next" song, which
greatly simplifies the cross-fading code.

src/buffer.h

@@ -0,0 +1,66 @@
+/*
+ * Copyright (C) 2003-2009 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+ */
+
+#ifndef MPD_MUSIC_BUFFER_H
+#define MPD_MUSIC_BUFFER_H
+
+/**
+ * An allocator for #music_chunk objects.
+ */
+struct music_buffer;
+
+/**
+ * Creates a new #music_buffer object.
+ *
+ * @param num_chunks the number of #music_chunk reserved in this
+ * buffer
+ */
+struct music_buffer *
+music_buffer_new(unsigned num_chunks);
+
+/**
+ * Frees the #music_buffer object
+ */
+void
+music_buffer_free(struct music_buffer *buffer);
+
+/**
+ * Returns the total number of reserved chunks in this buffer.  This
+ * is the same value which was passed to the constructor
+ * music_buffer_new().
+ */
+unsigned
+music_buffer_size(const struct music_buffer *buffer);
+
+/**
+ * Allocates a chunk from the buffer.  When it is not used anymore,
+ * call music_buffer_return().
+ *
+ * @return an empty chunk or NULL if there are no chunks available
+ */
+struct music_chunk *
+music_buffer_allocate(struct music_buffer *buffer);
+
+/**
+ * Returns a chunk to the buffer.  It can be reused by
+ * music_buffer_allocate() then.
+ */
+void
+music_buffer_return(struct music_buffer *buffer, struct music_chunk *chunk);
+
+#endif