add test for #5311 (average warping) and modify existing autotests for average and mode warping algorithms

git-svn-id: https://svn.osgeo.org/gdal/trunk@26695 f0d54148-0727-0410-94bb-9a71ac55c965
fix mode and near resampling corner computation (#5311)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26696 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx