Update to libpng 1.2.52 (rollup change only)

third_party/libpng/pngconf.h

Index: third_party/libpng/pngconf.h
diff --git a/third_party/libpng/pngconf.h b/third_party/libpng/pngconf.h
index 69282416157042a216f34db8756ab8c0ff446efd..008142e4dda1684d9e038d82b7f81e34aebb25f0 100644
--- a/third_party/libpng/pngconf.h
+++ b/third_party/libpng/pngconf.h
@@ -1,8 +1,8 @@
 
 /* pngconf.h - machine configurable file for libpng
  *
- * libpng version 1.2.45 - July 7, 2011
- * Copyright (c) 1998-2011 Glenn Randers-Pehrson
+ * libpng version 1.2.52 - November 20, 2014
+ * Copyright (c) 1998-2013 Glenn Randers-Pehrson
  * (Version 0.96 Copyright (c) 1996, 1997 Andreas Dilger)
  * (Version 0.88 Copyright (c) 1995, 1996 Guy Eric Schalnat, Group 42, Inc.)
  *
@@ -86,6 +86,18 @@
 #endif
 /* End of material added at libpng-1.2.19/1.2.21 */
 
+/* Added at libpng-1.2.51 (ported from 1.4.6) */
+#ifndef PNG_UNUSED
+/* Unused formal parameter warnings are silenced using the following macro
+ * which is expected to have no bad effects on performance (optimizing
+ * compilers will probably remove it entirely).  Note that if you replace
+ * it with something other than whitespace, you must include the terminating
+ * semicolon.
+ */
+#  define PNG_UNUSED(param) (void)param;
+#endif
+/* End of material added to libpng-1.4.6 */
+
 /* This is the size of the compression buffer, and thus the size of
  * an IDAT chunk.  Make this whatever size you feel is best for your
  * machine.  One of these will be allocated per png_struct.  When this
@@ -868,7 +880,7 @@
  * how large, set these two limits to 0.
  */
 #ifndef PNG_USER_CHUNK_CACHE_MAX
-#  define PNG_USER_CHUNK_CACHE_MAX 0
+#  define PNG_USER_CHUNK_CACHE_MAX 32765
 #endif
 
 /* Added at libpng-1.2.43 */