Update to libpng 1.6.2. We won't use the simplified API for now, as we want to be at least a little backward compatible with system libraries.
This commit is only vendor branch commit, merge with trunk will follow soon.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code@4550 dfc29bdd-3216-0410-991c-e03cc46cb475
Missed unversioned files

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code@4551 dfc29bdd-3216-0410-991c-e03cc46cb475
Merged libpng 1.6.2 from vendor branch. Updated pnglibconf.h to latest predefined config file to catch up with latest setup requirements.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code@4552 dfc29bdd-3216-0410-991c-e03cc46cb475