Avoid render warning - hiddenline background faces are not lit.

ObjectTypes/VolumeMeshObject/VolumeMeshNodeT_impl.hh

@@ -837,7 +837,7 @@ void VolumeMeshNodeT<VolumeMeshT>::getFaceRenderObjects(IRenderer* _renderer, GL
     if (_drawMode & (drawModes_.hiddenLineBackgroundFaces))
     {
         ro.shaderDesc.numLights = -1;
-        ro.shaderDesc.shadeMode = SG_SHADE_FLAT;
+        ro.shaderDesc.shadeMode = SG_SHADE_UNLIT;
 
         ro.shaderDesc.vertexColors = false;