Restore LIBPNG for png files

Restores the use of libpng for png files as stb does not support writing out 16bit png files.

Engine/lib/lpng/scripts/prefix.c

@@ -0,0 +1,24 @@
+
+/* prefix.c - generate an unprefixed symbol list
+ *
+ * Last changed in libpng version 1.6.16 [December 22, 2014]
+ * Copyright (c) 2013-2014 Glenn Randers-Pehrson
+ *
+ * This code is released under the libpng license.
+ * For conditions of distribution and use, see the disclaimer
+ * and license in png.h
+ */
+
+#define PNG_EXPORTA(ordinal, type, name, args, attributes)\
+        PNG_DFN "@" name "@"
+
+/* The configuration information *before* the additional of symbol renames,
+ * the list is the C name list; no symbol prefix.
+ */
+#include "pnglibconf.out"
+
+PNG_DFN_START_SORT 1
+
+#include "../png.h"
+
+PNG_DFN_END_SORT