CopyWholeRaster(): make default GDAL_SWATH_SIZE to 1/4 of GDAL_CACHEMAX instead of hard-coded value of 10 MB

git-svn-id: https://svn.osgeo.org/gdal/trunk@26896 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx