[png-mng-implement] libpng-1.6.31beta06 is available

[Glenn Randers-Pehrson]

libpng-1.6.31beta06 is available from
https://ftp-osl.osuosl.org/pub/libpng/src/beta/libpng16
ftp://ftp-osl.osuosl.org/pub/libpng/src/beta/libpng16
http://libpng.download/src/beta/libpng16
and from
http://libpng.sf.net

Version 1.6.31beta06 [July 17, 2017]
Added a minimal eXIf chunk (with Orientation and FocalLengthIn35mmFilm
tags) to pngtest.png.

Glenn