Added support for float or double osg::Plane, and the default osg::Plane to double.

Performance tests on big models did not indicate any performance penalty in using doubles over floats,
so the move to doubles should mainly impact precision improvements for whole earth databases.

Also made improvements to osgUtil::PlaneIntersector and osgSim::ElevationSlice classes

examples/osgtexture3D/osgtexture3D.cpp

@@ -89,7 +89,7 @@ class ConstructStateCallback : public osgProducer::OsgCameraGroup::RealizeCallba
             // will do the blending for us.
             osg::TexGen* texgen = new osg::TexGen;
             texgen->setMode(osg::TexGen::OBJECT_LINEAR);
-            texgen->setPlane(osg::TexGen::R, osg::Vec4(0.0f,0.0f,0.0f,0.2f));
+            texgen->setPlane(osg::TexGen::R, osg::Plane(0.0f,0.0f,0.0f,0.2f));
 
             // create the StateSet to store the texture data
             osg::StateSet* stateset = new osg::StateSet;