envelope() method added to DelaunayTriangulationBuilder class

Includes testcase. Patch by Vishal Tiwari

git-svn-id: http://svn.osgeo.org/geos/trunk@3862 5242fede-7e19-0410-aef8-94bd7d2200fb
Simplify testcase for DelaunayTriangulationBuilder::envelope

git-svn-id: http://svn.osgeo.org/geos/trunk@3863 5242fede-7e19-0410-aef8-94bd7d2200fb
Avoid Coordinate copies in DelaunayTriangulationBuilder::envelope

git-svn-id: http://svn.osgeo.org/geos/trunk@3864 5242fede-7e19-0410-aef8-94bd7d2200fb