Update to libpng 1.6.23 (from 1.6.21).

Vendors and 1.8 branch updated.


git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@5314 dfc29bdd-3216-0410-991c-e03cc46cb475

tests/tests-last-passed-at.txt

-Tests finished. 2 tests of 2 passed.
-Compiled as DEBUG
-Test suite pass at GMT Fri Oct 16 08:43:01 2015
-
+Tests finished. 2 tests of 2 passed.
+Compiled as DEBUG
+Test suite pass at GMT Fri Jul  8 21:06:13 2016
+