Added support for AbortRendering flag pointer in osg::State which is used

by osgUtil::RenderLeaf to test if any abort state has been set, if so it
doesn't do any further drawing.

The osgProducer::Viewer has been set up to set the osg::State's it manages
with their AbortRendering flag pointers set to the osgProducer::Viewer::_done
memeber varaible.  Now when escape is pressed the rendering is aborted early.

src/osgUtil/RenderLeaf.cpp

@@ -18,6 +18,12 @@ using namespace osgUtil;
 
 void RenderLeaf::render(State& state,RenderLeaf* previous)
 {
+    // don't draw this leaf if the abort rendering flag has been set.
+    if (state.getAbortRendering())
+    {
+        //cout << "early abort"<<endl;
+        return;
+    }
 
     if (previous)
     {