Average/mode kernels of warp: make them less sensitive to numerical precision issues (should fix failures on MinGW32) (patch by Etienne Tourigny, #5350)

git-svn-id: https://svn.osgeo.org/gdal/trunk@26846 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx