Added experimental image 3d downsample function.
Added experimental image 3d downsample function.


XiaoCC/osg,XiaoCC/osg,ghub/osg,omega-hub/osg,ntoand/osg,artoolkit/osg,koshman86/osg,artoolkit/osg,koshman86/osg,artoolkit/osg,XiaoCC/osg,omega-hub/osg,ghub/osg,ntoand/osg,ghub/osg,omega-hub/osg,koshman86/osg,omega-hub/osg,artoolkit/osg,ntoand/osg,XiaoCC/osg,ntoand/osg,XiaoCC/osg,ghub/osg,koshman86/osg