[libpng16] Revised png_set_keep_unknown_chunks() so num_chunks < 0 means

ignore all unknown chunks and all known chunks except for IHDR, PLTE, tRNS,
IDAT, and IEND.  Previously it only meant ignore all unknown chunks, the
same as num_chunks == 0. Revised png_image_skip_unused_chunks() to
provide a list of chunks to be processed instead of a list of chunks to
ignore.  Revised contrib/gregbook/readpng2.c accordingly.
diff --git a/CHANGES b/CHANGES
index 52a60a0..a08218d 100644
--- a/CHANGES
+++ b/CHANGES
@@ -4136,7 +4136,13 @@
 Version 1.6.0beta24 [June 7, 2012]
   Don't check palette indexes if num_palette is 0 (as it can be in MNG files).
 
-Version 1.6.0beta25 [June 12, 2012]
+Version 1.6.0beta25 [June 16, 2012]
+  Revised png_set_keep_unknown_chunks() so num_chunks < 0 means ignore all
+    unknown chunks and all known chunks except for IHDR, PLTE, tRNS, IDAT,
+    and IEND.  Previously it only meant ignore all unknown chunks, the
+    same as num_chunks == 0. Revised png_image_skip_unused_chunks() to
+    provide a list of chunks to be processed instead of a list of chunks to
+    ignore.  Revised contrib/gregbook/readpng2.c accordingly.
 
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