[png-mng-implement] libpng-1.6.29rc01 is available

[Glenn Randers-Pehrson]

libpng-1.6.29rc01 is available from
ftp://ftp.simplesystems.org/pub/png-group/src/libpng16/
and from
http://libpng.sf.net

There are no changes from version 1.6.29beta03.
If no problems are reported, I'll release libpng-1.6.29
on March 16, 2017.

Glenn