GeoJSON writer: fix sometimes incorrect result (significant digit losts...) when using -lco COORDINATE_PRECISION=0; actually fix it in common OGRFormatDouble() and remove GeoJSON duplicated implementation

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@26807 f0d54148-0727-0410-94bb-9a71ac55c965