Codechange: mark chunks that are not stored as CH_READONLY

This makes it easier to spot chunks that have a save_proc that
is a nullptr, but also prevents confusion, where it looks like
the CH_ type of a chunk has influence on how it is being read.
It is not, it is only used for saving.

src/saveload/saveload.h

@@ -388,6 +388,7 @@ enum ChunkType {
 	CH_RIFF = 0,
 	CH_ARRAY = 1,
 	CH_SPARSE_ARRAY = 2,
+	CH_READONLY, ///< Chunk is never saved.
 };
 
 /** Handlers and description of chunk. */