[png-mng-implement] libpng-1.2.36beta01 and libpng-1.4.0beta49 are available

[Glenn Randers-P. <gl...@gm...>]

libpng-1.2.36beta01 and libpng-1.4.0beta49 are available from
ftp://ftp.simplesystems.org/pub/png-group/src
and from
http://libpng.sf.net

The order of #ifdef's in the png_debug definition is fixed.

In version 1.4.0beta49, an incorrect argument list in the
png_memset() call for pcal params is fixed.
Some comments in pngrio.c and pngwio.c are revised.

There is no change to png_default_flush() because I did not
know how to make it look more portable.  Maybe "fileno != -1"
could be changed to "fileno >= 0" if that would help.

Glenn