warper: regardless of the warping memory limit, add heuristics to determine if we must split the target window in case the 'fill ratio' of the source dataset is too low (#3120)

git-svn-id: https://svn.osgeo.org/gdal/trunk@28258 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_vc12