Envisat: implement more reliable way of extracting GCPs from Meris tie-points; add DEM corrections of TP-ADS products when present; workaround dateline discontinuity in GCPs so they can be used with GDAL warping transformers (patch by Martin Paces, #5423)

git-svn-id: https://svn.osgeo.org/gdal/trunk@27098 f0d54148-0727-0410-94bb-9a71ac55c965