OverlayOp: restrict intersection computations to the target extent

Speeds up INTERSECTION and DIFFERENCE operations between geometries
with small bounding box overlap.

git-svn-id: http://svn.osgeo.org/geos/trunk@4017 5242fede-7e19-0410-aef8-94bd7d2200fb
Further speedup difference/intersection at the validating phase

This avoids to insert to the output graph edges which do not intersect
the target envelope.

git-svn-id: http://svn.osgeo.org/geos/trunk@4018 5242fede-7e19-0410-aef8-94bd7d2200fb