Update to libpng 1.6.24 (#807)

thirdparty/libpng/LICENSE

@@ -10,7 +10,7 @@ this sentence.
 
 This code is released under the libpng license.
 
-libpng versions 1.0.7, July 1, 2000, through 1.6.21, January 15, 2016, are
+libpng versions 1.0.7, July 1, 2000 through 1.6.24, August 4, 2016 are
 Copyright (c) 2000-2002, 2004, 2006-2016 Glenn Randers-Pehrson, are
 derived from libpng-1.0.6, and are distributed according to the same
 disclaimer and license as libpng-1.0.6 with the following individuals
@@ -32,6 +32,10 @@ and with the following additions to the disclaimer:
  risk of satisfactory quality, performance, accuracy, and effort is with
  the user.
 
+Some files in the "contrib" directory and some configure-generated
+files that are distributed with libpng have other copyright owners and
+are released under other open source licenses.
+
 libpng versions 0.97, January 1998, through 1.0.6, March 20, 2000, are
 Copyright (c) 1998-2000 Glenn Randers-Pehrson, are derived from
 libpng-0.96, and are distributed according to the same disclaimer and
@@ -55,6 +59,9 @@ Contributing Authors:
  Greg Roelofs
  Tom Tanner
 
+Some files in the "scripts" directory have other copyright owners
+but are released under this license.
+
 libpng versions 0.5, May 1995, through 0.88, January 1996, are
 Copyright (c) 1995-1996 Guy Eric Schalnat, Group 42, Inc.
 
@@ -95,18 +102,29 @@ appreciated.
 
 END OF COPYRIGHT NOTICE, DISCLAIMER, and LICENSE.
 
-A "png_get_copyright" function is available, for convenient use in "about"
-boxes and the like:
+TRADEMARK:
 
- printf("%s", png_get_copyright(NULL));
+The name "libpng" has not been registered by the Copyright owner
+as a trademark in any jurisdiction. However, because libpng has
+been distributed and maintained world-wide, continually since 1995,
+the Copyright owner claims "common-law trademark protection" in any
+jurisdiction where common-law trademark is recognized.
 
-Also, the PNG logo (in PNG format, of course) is supplied in the
-files "pngbar.png" and "pngbar.jpg (88x31) and "pngnow.png" (98x31).
+OSI CERTIFICATION:
 
 Libpng is OSI Certified Open Source Software. OSI Certified Open Source is
 a certification mark of the Open Source Initiative. OSI has not addressed
 the additional disclaimers inserted at version 1.0.7.
 
+EXPORT CONTROL:
+
+The Copyright owner believes that the Export Control Classification
+Number (ECCN) for libpng is EAR99, which means not subject to export
+controls or International Traffic in Arms Regulations (ITAR) because
+it is open source, publicly available software, that does not contain
+any encryption software. See the EAR, paragraphs 734.3(b)(3) and
+734.7(b).
+
 Glenn Randers-Pehrson
 glennrp at users.sourceforge.net
-January 15, 2016
+August 4, 2016