[libpng16] Catch up with some recent changes that did not get merged.

pngrutil.c

@@ -2766,6 +2766,7 @@ png_handle_unknown(png_structrp png_ptr, png_inforp info_ptr,
 
    png_debug(1, "in png_handle_unknown");
 
+#ifdef PNG_READ_UNKNOWN_CHUNKS_SUPPORTED
    /* NOTE: this code is based on the code in libpng-1.4.12 except for fixing
     * the bug which meant that setting a non-default behavior for a specific
     * chunk would be ignored (the default was always used unless a user
@@ -2778,14 +2779,15 @@ png_handle_unknown(png_structrp png_ptr, png_inforp info_ptr,
     * function.
     */
 #  ifndef PNG_HANDLE_AS_UNKNOWN_SUPPORTED
-   keep = PNG_HANDLE_CHUNK_AS_DEFAULT;
+#     ifdef PNG_SET_UNKNOWN_CHUNKS_SUPPORTED
+         keep = png_chunk_unknown_handling(png_ptr, png_ptr->chunk_name);
+#     endif
 #  endif
 
    /* One of the following methods will read the chunk or skip it (at least one
     * of these is always defined because this is the only way to switch on
     * PNG_READ_UNKNOWN_CHUNKS_SUPPORTED)
     */
-#ifdef PNG_READ_UNKNOWN_CHUNKS_SUPPORTED
 #  ifdef PNG_READ_USER_CHUNKS_SUPPORTED
       /* The user callback takes precedence over the chunk keep value, but the
        * keep value is still required to validate a save of a critical chunk.
@@ -2893,7 +2895,7 @@ png_handle_unknown(png_structrp png_ptr, png_inforp info_ptr,
 
          png_crc_finish(png_ptr, length);
       }
-#  endif /* PNG_SAVE_UNKNOWN_CHUNKS_SUPPORTED */
+#  endif
 
 #  ifdef PNG_STORE_UNKNOWN_CHUNKS_SUPPORTED
       /* Now store the chunk in the chunk list if appropriate, and if the limits
@@ -2932,9 +2934,8 @@ png_handle_unknown(png_structrp png_ptr, png_inforp info_ptr,
          }
 #     endif
       }
-#  else /* no store support! */
+#  else /* no store support: the chunk must be handled by the user callback */
       PNG_UNUSED(info_ptr)
-#     error untested code (reading unknown chunks with no store support)
 #  endif
 
    /* Regardless of the error handling below the cached data (if any) can be