Merge the new libpng version. Seems to take care of the WIN_CE problems on its own, so no need for the 'if 0' check anymore. Also fixes the const static warnings.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@1400 dfc29bdd-3216-0410-991c-e03cc46cb475