Simplified the API for using the DatabasePager, by providing a single

DatabasePager::updateSceneGraph(..) method, and added a ref_ptr<> into
osDB::Registry for managing a single DatabasePager in a centralised way.

src/osgDB/Registry.cpp

@@ -197,6 +197,11 @@ Registry::Registry()
 
 Registry::~Registry()
 {
+    // switch off the pager and its associated thread before we clean up 
+    // rest of the Registry.
+    _databasePager = 0;
+
+    // unload all the plugin before we finally destruct.
     closeAllLibraries();
 }
 
@@ -1835,3 +1840,10 @@ void Registry::clearObjectCache()
 {
     _objectCache.clear();
 }
+
+DatabasePager* Registry::getOrCreateDatabasePager()
+{
+    if (!_databasePager) _databasePager = new DatabasePager;
+    
+    return _databasePager.get();
+}