Added +png-color-type-rgba+ constant, added preserve-alpha parameter to decode-file, updated encode to handle rgba images.
Remove +png-libpng-ver-string+ because it doesn't compile.
add support for fresh libpng versions
CL-PNG is a Common Lisp package for reading and writing PNG (Portable Network Graphics) files. The CL-PNG homepage is at <URL:http://www.ljosa.com/~ljosa/software/cl-png/>. Copyright (C) 2001--2009 by the authors (see the file AUTHORS). Licensed under the GNU Lesser General Public License; see the file COPYING for details. See doc/index.html for documentation.