Added explicit initialization of osg::Referenced(true) to osg::Operation subclasses as it uses virtual inhertiance from osg::Referenced.

git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@15025 16af8721-9629-0410-8352-f15c8da7e697

src/osg/OperationThread.cpp

@@ -22,6 +22,7 @@ using namespace OpenThreads;
 struct BlockOperation : public Operation, public Block
 {
     BlockOperation():
+        osg::Referenced(true),
         Operation("Block",false)
     {
         reset();