Merged latest libpng upgrade in vendor/libpng to cafu/trunk/ExtLibs/libpng.
Note that this implements the "Vendor Branches" strategy described in the Subversion book at http://svnbook.red-bean.com/nightly/en/svn.branchmerge.basicmerging.html http://svnbook.red-bean.com/nightly/en/svn.advanced.vendorbr.html