wesnoth-optipng pass

Overall statistics (only for files with a smaller recompressed size):
  Original size: 15583 KiB on 283 files
 Optimized size: 11794 KiB
   Total saving: 3788 KiB = 24% decrease

git-svn-id: http://svn.gna.org/svn/wesnoth/trunk@56565 75b3775d-3801-0410-9e12-e2e43121521d