Exposes Boolean for D3D rendering in DesignMode as DependencyProperty

So one can switch rendering in VS-Desingner or Blend. This is usefull as D3D rendering by itself in DesignMode was not cause of the failure and makes sense in most cases. But CubeRenderer, and propably some other bad D3D would cause the Designer to crash. In case you dont have bad D3D, one can enable still rendering if needed.