Added support for specifying whether view frustum and small feature culling

should be applied to a node or its child with the new osg::Node::setCullingActive()
flag.  A mechanism has been implemented so that if child has its culling disabled
then their parents, all the way up to the root are also have their culling
implicitly disabled.

The osg::CullVisitor has updated to take account of
both the explicit control via setCullingActive and the implicit culling
disabling through children being disabled.

This feature is useful for nodes which don't have a bounding volume to cull
against, earth sky implementations and light sources.

The default osg::Node::_cullingActive is true, i.e. culling is enabled by
default.

src/osg/Group.cpp

@@ -62,6 +62,16 @@ bool Group::addChild( Node *child )
                 );
         }
 
+        // could now require disabling of culling thanks to the new subgraph,
+        // so need to check and update if required.
+        if (child->getNumChildrenWithCullingDisabled()>0 ||
+            !child->getCullingActive())
+        {
+            setNumChildrenWithCullingDisabled(
+                getNumChildrenWithCullingDisabled()+1
+                );
+        }
+
         return true;
     }
     else return false;
@@ -89,6 +99,14 @@ bool Group::removeChild( Node *child )
                 );
         }
 
+        if (child->getNumChildrenWithCullingDisabled()>0 ||
+            !child->getCullingActive())
+        {
+            setNumChildrenWithCullingDisabled(
+                getNumChildrenWithCullingDisabled()-1
+                );
+        }
+
         // note ref_ptr<> automatically handles decrementing child's reference count.
         _children.erase(itr);
         dirtyBound();
@@ -118,6 +136,50 @@ bool Group::replaceChild( Node *origNode, Node *newNode )
         newNode->_parents.push_back(this);
 
         dirtyBound();
+
+
+        // could now require app traversal thanks to the new subgraph,
+        // so need to check and update if required.
+        int delta_numChildrenRequiringAppTraversal = 0;
+        if (origNode->getNumChildrenRequiringAppTraversal()>0 ||
+            origNode->getAppCallback())
+        {
+            --delta_numChildrenRequiringAppTraversal;
+        }
+        if (newNode->getNumChildrenRequiringAppTraversal()>0 ||
+            newNode->getAppCallback())
+        {
+            ++delta_numChildrenRequiringAppTraversal;
+        }
+
+        if (delta_numChildrenRequiringAppTraversal!=0)
+        {
+            setNumChildrenRequiringAppTraversal(
+                getNumChildrenRequiringAppTraversal()+delta_numChildrenRequiringAppTraversal
+                );
+        }
+
+        // could now require disabling of culling thanks to the new subgraph,
+        // so need to check and update if required.
+        int delta_numChildrenWithCullingDisabled = 0;
+        if (origNode->getNumChildrenWithCullingDisabled()>0 ||
+            !origNode->getCullingActive())
+        {
+            --delta_numChildrenWithCullingDisabled;
+        }
+        if (newNode->getNumChildrenWithCullingDisabled()>0 ||
+            !newNode->getCullingActive())
+        {
+            ++delta_numChildrenWithCullingDisabled;
+        }
+        
+        if (delta_numChildrenWithCullingDisabled!=0)
+        {
+            setNumChildrenWithCullingDisabled(
+                getNumChildrenWithCullingDisabled()-1
+                );
+        }
+
         return true;
     }
     else return false;