Added collateReferencesToDependentCameras() and clearReferencesToDependentCameras() methods into RenderStage and SceneView, and use

of these methods in src/osgViewer/Renderer.cpp to make sure that the draw thread keeps references to all in scene graph Cameras
that are being used by the drawing threads, to keep the Camera's alive even when the main thread removes these Cameras from the scene graph.

src/osgViewer/Renderer.cpp

@@ -384,6 +384,11 @@ void Renderer::draw()
 
     if (sceneView && !_done)
     {
+        // since we are running the draw thread in parallel with the main thread it's possible to unreference Camera's
+        // that are still being used by this rendering thread, so to prevent this we'll take references to all these
+        // Camera's and the clear these references once we've completed the whole draw dispatch.
+        sceneView->collateReferencesToDependentCameras();
+
         if (_compileOnNextDraw)
         {
             compile();
@@ -485,6 +490,7 @@ void Renderer::draw()
             stats->setAttribute(frameNumber, "Draw traversal time taken", osg::Timer::instance()->delta_s(beforeDrawTick, afterDrawTick));
         }
 
+        sceneView->clearReferencesToDependentCameras();
     }
 
     DEBUG_MESSAGE<<"end draw() "<<this<<std::endl;