libpng: update to upstream version 1.5.13

libpng: update to upstream version 1.5.13

Updating from 1.2.x means also change in actual library name
from libpng12 to libpng15, and slight changes in API.

License md5sums differ only because of license text formatting
changes and because they contain version numbers and release dates.

(From OE-Core rev: 317cbe5f5d4bf9366657a0d5383108c1b3340a8a)

Signed-off-by: Marko Lindqvist <cazfi74-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>