MusicChunk: pad MusicChunkInfo to a multiple of 8 bytes

Workaround for a regression caused by commit
a06bf388d9, revealing a problem with
discarding odd numer of frames in the DSD_U32 and DoP converters,
causing distortions with DSD_U32 and DoP on 32 bit CPUs.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/469

NEWS

@@ -1,4 +1,6 @@
 ver 0.21.11 (not yet released)
+* output
+  - alsa, osx: fix distortions with DSD_U32 and DoP on 32 bit CPUs
 * protocol
   - fix "list" with multiple "group" levels
 

src/MusicChunk.hxx

@@ -43,7 +43,15 @@ struct MusicChunk;
 /**
  * Meta information for #MusicChunk.
  */
-struct MusicChunkInfo {
+struct alignas(8) MusicChunkInfo {
+	/* align to multiple of 8 bytes, which adds padding at the
+	   end, so the size of MusicChunk::data is also a multiple of
+	   8 bytes; this is a workaround for a bug in the DSD_U32 and
+	   DoP converters which require processing 8 bytes at a time,
+	   discarding the remainder */
+	/* TODO: once all converters have been fixed, we should remove
+	   this workaround */
+
 	/** the next chunk in a linked list */
 	MusicChunkPtr next;
 
@@ -119,6 +127,10 @@ struct MusicChunk : MusicChunkInfo {
 	/** the data (probably PCM) */
 	uint8_t data[CHUNK_SIZE - sizeof(MusicChunkInfo)];
 
+	/* TODO: remove this check once all converters have been fixed
+	   (see comment in struct MusicChunkInfo for details) */
+	static_assert(sizeof(data) % 8 == 0, "Wrong alignment");
+
 	/**
 	 * Prepares appending to the music chunk.  Returns a buffer
 	 * where you may write into.  After you are finished, call