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/osg/State.cpp

@@ -23,7 +23,8 @@ State::State()
     _identity = new osg::RefMatrix(); // default RefMatrix constructs to identity.
     _projection = _identity;
     _modelView = _identity;
-    
+
+    _abortRenderingPtr = false;    
     _reportGLErrors = true;
 
     _currentActiveTextureUnit=0;