update libpng

update libpng, the repo now requires a vcpkg setup for integrating but skipping the install step should allow it to work for windows an linux, mac might need more

Engine/lib/lpng/contrib/testpngs/makepngs.sh

@@ -1,12 +1,10 @@
-#!/bin/sh
-#
+#!/usr/bin/env bash
+
 # Make a set of test PNG files, MAKEPNG is the name of the makepng executable
 # built from contrib/libtests/makepng.c
 
 # Copyright (c) 2015 John Cunningham Bowler
 
-# Last changed in libpng 1.6.20 [December 3, 2015]
-
 # This code is released under the libpng license.
 # For conditions of distribution and use, see the disclaimer
 # and license in png.h