Upgraded PNG library to version 1.6.17

nickgammon · nickgammon · commit e995b2c179d5 · 2015-04-07T10:29:26.000+10:00
diff --git a/png/readme.txt b/png/readme.txt
@@ -2,7 +2,7 @@ PNG built as follows:
 
 1. Download recent version from download.sourceforge.net
 
-    In this case: lpng1615.zip 
+    In this case: ftp://ftp.simplesystems.org/pub/libpng/png/src/libpng16/lpng1617.zip
 
 2. Uncompress the archive with 7-Zip (if necessary), obtainable from:
 
