MusicPipe: eliminate the unused MusicBuffer reference

This requires re-adding the reference to struct DecoderControl, which
was removed recently by commit
9f14e7a98d

src/decoder/Bridge.cxx

@@ -98,7 +98,7 @@ DecoderBridge::GetChunk() noexcept
 		return current_chunk.get();
 
 	do {
-		current_chunk = dc.pipe->GetBuffer().Allocate();
+		current_chunk = dc.buffer->Allocate();
 		if (current_chunk != nullptr) {
 			current_chunk->replay_gain_serial = replay_gain_serial;
 			if (replay_gain_serial != 0)

src/decoder/DecoderControl.cxx

@@ -92,7 +92,7 @@ DecoderControl::IsCurrentSong(const DetachedSong &_song) const noexcept
 void
 DecoderControl::Start(std::unique_ptr<DetachedSong> _song,
 		      SongTime _start_time, SongTime _end_time,
-		      MusicPipe &_pipe) noexcept
+		      MusicBuffer &_buffer, MusicPipe &_pipe) noexcept
 {
 	assert(_song != nullptr);
 	assert(_pipe.IsEmpty());
@@ -100,6 +100,7 @@ DecoderControl::Start(std::unique_ptr<DetachedSong> _song,
 	song = std::move(_song);
 	start_time = _start_time;
 	end_time = _end_time;
+	buffer = &_buffer;
 	pipe = &_pipe;
 
 	ClearError();

src/decoder/DecoderControl.hxx

@@ -44,6 +44,7 @@
 #endif
 
 class DetachedSong;
+class MusicBuffer;
 class MusicPipe;
 
 enum class DecoderState : uint8_t {
@@ -151,6 +152,9 @@ struct DecoderControl final : InputStreamHandler {
 
 	SignedSongTime total_time;
 
+	/** the #MusicChunk allocator */
+	MusicBuffer *buffer;
+
 	/**
 	 * The destination pipe for decoded chunks.  The caller thread
 	 * owns this object, and is responsible for freeing it.
@@ -379,7 +383,7 @@ public:
 	 */
 	void Start(std::unique_ptr<DetachedSong> song,
 		   SongTime start_time, SongTime end_time,
-		   MusicPipe &pipe) noexcept;
+		   MusicBuffer &buffer, MusicPipe &pipe) noexcept;
 
 	/**
 	 * Caller must lock the object.