[png-mng-implement] libpng-1.2.37beta01 and libpng-1.4.0beta58 are available

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

libpng-1.2.37beta01 and libpng-1.4.0beta58 are available from
ftp://ftp.simplesystems.org/pub/png-group/src/
and from
http://libpng.sf.net/index.html

In libpng-1.2.37beta01 the "png_ptr->rowbytes" parameter to
the memset() call is changed to "row_bytes" to be consistent
with the corresponding png_malloc().  Some of the makefiles
are merged with the newer libpng-1.4.0beta58 makefiles.

In both, the comments in example.c are reformatted and a
comment giving advice about setting pointers to NULL after
png_free() is added, and incorrect use of the sig_bit struct
is corrected in example.c

Glenn