Added support for osg::StateSet comparison operators and using this new feature

added support in osgUtil::OptimizeStateVisitor for removing duplicate
StateSet's from the scene graph, previously only duplicated StateAttributes
we're removed.

src/osg/StateSet.cpp

@@ -29,6 +29,50 @@ StateSet::~StateSet()
     // delete the memory manually.
 }
 
+int StateSet::compare(const StateSet& rhs) const
+{
+    // check to see how the modes compare.
+    ModeList::const_iterator lhs_mode_itr = _modeList.begin();
+    ModeList::const_iterator rhs_mode_itr = rhs._modeList.begin();
+    while (lhs_mode_itr!=_modeList.end() && rhs_mode_itr!=rhs._modeList.end())
+    {
+        if      (lhs_mode_itr->first<rhs_mode_itr->first) return -1;
+        else if (rhs_mode_itr->first<lhs_mode_itr->first) return 1;
+        if      (lhs_mode_itr->second<rhs_mode_itr->second) return -1;
+        else if (rhs_mode_itr->second<lhs_mode_itr->second) return 1;
+        ++lhs_mode_itr;
+        ++rhs_mode_itr;
+    }
+    if (lhs_mode_itr==_modeList.end())
+    {
+        if (rhs_mode_itr!=rhs._modeList.end()) return -1;
+    }
+    else if (rhs_mode_itr == rhs._modeList.end()) return 1;
+
+    // we've got here so modes must be equal...
+    
+    // now check to see how the attributes compare.
+    AttributeList::const_iterator lhs_attr_itr = _attributeList.begin();
+    AttributeList::const_iterator rhs_attr_itr = rhs._attributeList.begin();
+    while (lhs_attr_itr!=_attributeList.end() && rhs_attr_itr!=rhs._attributeList.end())
+    {
+        if      (lhs_attr_itr->first<rhs_attr_itr->first) return -1;
+        else if (rhs_attr_itr->first<lhs_attr_itr->first) return 1;
+        if      (lhs_attr_itr->second.first<rhs_attr_itr->second.first) return -1;
+        else if (rhs_attr_itr->second.first<lhs_attr_itr->second.first) return 1;
+        if      (lhs_attr_itr->second.second<rhs_attr_itr->second.second) return -1;
+        else if (rhs_attr_itr->second.second<lhs_attr_itr->second.second) return 1;
+        ++lhs_attr_itr;
+        ++rhs_attr_itr;
+    }
+    if (lhs_attr_itr==_attributeList.end())
+    {
+        if (rhs_attr_itr!=rhs._attributeList.end()) return -1;
+    }
+    else if (rhs_attr_itr == rhs._attributeList.end()) return 1;
+
+    return 0;
+}
 
 void StateSet::setGlobalDefaults()
 {