Made the LineSegment destructor protected to force users to create segments

on the stack thus ensure that they arn't created locally and have their
memory deleted incorrectly.

Also updated the IntersectVisitor so that it no used osg::ref_ptr<> internally
for storing all data. Have also move the IntersectState helper class to inside the
IntersectVisitor which simplifies the external interface to the class.

include/osg/LineSegment

@@ -19,7 +19,6 @@ class SG_EXPORT LineSegment : public Referenced
         LineSegment() {};
         LineSegment(const LineSegment& seg) : Referenced(),_s(seg._s),_e(seg._e) {}
         LineSegment(const Vec3& s,const Vec3& e) : _s(s),_e(e) {}
-        virtual ~LineSegment() {}
 
         LineSegment& operator = (const LineSegment& seg) { _s = seg._s;  _e = seg._e; return *this; }
 
@@ -55,6 +54,8 @@ class SG_EXPORT LineSegment : public Referenced
 
     protected:
 
+        virtual ~LineSegment() {}
+        
         static const bool intersectAndClip(Vec3& s,Vec3& e,const BoundingBox& bb);
 
         Vec3 _s;