gdal_translate: use floor() to compute image coordinates from world coordinates when specifying -projwin (usefull when extracting from left or top of upper-left corner, which generate negative image coordinates) (#5367)

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26894 f0d54148-0727-0410-94bb-9a71ac55c965