Improve overlay robustness

 - Validate CBR results before accepting them
 - Enable overlay node validator even for FIXED precision
 - Enable geometry-reduction policy
 - Bail out on exception from overlay if any input is invalid

Fixes bug #459

git-svn-id: http://svn.osgeo.org/geos/trunk@3853 5242fede-7e19-0410-aef8-94bd7d2200fb