Added support for setting up VisualChoose with destination alpha, stencil and

quad buffers attributes from DisplaySettings values.

examples/osgprerender/osgprerender.cpp

@@ -103,7 +103,7 @@ void MyCullCallback::doPreRender(osg::Node&, osgUtil::CullVisitor& cv)
     osgUtil::RenderStage* previous_stage = cv.getCurrentRenderBin()->_stage;
 
     // set up the background color and clear mask.
-    rtts->setClearColor(osg::Vec4(0.1f,0.1f,0.3f,0.0f));
+    rtts->setClearColor(osg::Vec4(0.1f,0.1f,0.3f,1.0f));
     rtts->setClearMask(previous_stage->getClearMask());
 
     // set up to charge the same RenderStageLighting is the parent previous stage.