Fix gdal/data/gdalicon.png and autotest/gdrivers/data/test.png to be read by libpng 1.6 and zlib 1.2.4 or above

git-svn-id: https://svn.osgeo.org/gdal/trunk@26597 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx