Refactored the way that the DatabasePager passes the Terrain decorator node onto the TerrainTile.

The DatabasePager now passes the Terrain pointer into the ReaderWriter's via osgDB::Options object,
rather than pushing a NodePath containing the Terrain onto NodeVisitor.  This
change means that the DatabasePager nolonger needs to observer the whole NodePath and
will be lighter and quicker for it.

The change also means that ReadFileCallback can now run custom NodeVisitor's on the scene graph without
having to worry about TerrainTile's constructing scene graphs prior to the Terrain being assigned.

Also changed is the NodeVisitor::DatabaseRequestHandler which now requires a NodePath to the node that you wish
to add to rather than just the pointer to the node you wish to add to.  This is more robust when handling scenes
with multiple parental paths, whereas previously errors could have occurred due to the default of picking the first
available parental path.  This change means that subclasses of DatabasePager will need to be updated to use this new
function entry point.

include/osg/Node

@@ -23,8 +23,15 @@
 #include <string>
 #include <vector>
 
+
+// forward declare osgTerrrain::Terrain to enable declaration of asTerrain() method.
+namespace osgTerrain {
+class Terrain;
+}
+
 namespace osg {
 
+// forcing declare classes to enable declaration of as*() methods.
 class NodeVisitor;
 class Group;
 class Transform;
@@ -114,6 +121,15 @@ class OSG_EXPORT Node : public Object
           * Equivalent to dynamic_cast<const Geode*>(this).*/
         virtual const Geode* asGeode() const { return 0; }
 
+        /** Convert 'this' into a Transform pointer if Node is a Terrain, otherwise return 0.
+          * Equivalent to dynamic_cast<Terrrain*>(this).*/
+        virtual osgTerrain::Terrain* asTerrain() { return 0; }
+
+        /** convert 'const this' into a const Terrain pointer if Node is a Terrain, otherwise return 0.
+          * Equivalent to dynamic_cast<const Terrain*>(this).*/
+        virtual const osgTerrain::Terrain* asTerrain() const { return 0; }
+
+
         /** Visitor Pattern : calls the apply method of a NodeVisitor with this node's type.*/
         virtual void accept(NodeVisitor& nv);
         /** Traverse upwards : calls parents' accept method with NodeVisitor.*/