Fix GEOSVoronoiDiagram on requesting edges only

Before this fix asking for edges would return the edges of
the triangle used as abase for the Voronoi diagram.
After, we return the actual voronoi cell edges.

git-svn-id: http://svn.osgeo.org/geos/trunk@3983 5242fede-7e19-0410-aef8-94bd7d2200fb
Expose Geometry.voronoiDiagram in PHP interface

git-svn-id: http://svn.osgeo.org/geos/trunk@3984 5242fede-7e19-0410-aef8-94bd7d2200fb