GDALRasterBand::GetDefaultHistogram(): change how min and max bounds are computed in the non GDT_Byte case

git-svn-id: https://svn.osgeo.org/gdal/trunk@25856 f0d54148-0727-0410-94bb-9a71ac55c965
ECW: take account of half bucket with to alter slightly the min and max values returned by GetDefaultHistogram(); test this API

git-svn-id: https://svn.osgeo.org/gdal/trunk@25857 f0d54148-0727-0410-94bb-9a71ac55c965