change "bytes" parameter of all read callbacks from "unsigned" to "size_t"

include/FLAC/all.h

@@ -312,6 +312,10 @@
  * two new convenience functions that may be useful:
  * FLAC__metadata_object_cuesheet_calculate_cddb_id() and
  * FLAC__metadata_get_cuesheet().
+ *
+ * The \a bytes parameter to FLAC__StreamDecoderReadCallback and
+ * FLAC__StreamEncoderReadCallback is now \c size_t instead of
+ * \c unsigned.
  */
 
 /** \defgroup flac FLAC C API