enabled twosidedlighting for face coloring with flat shading

ObjectTypes/VolumeMeshObject/VolumeMeshNodeT.cc

@@ -656,6 +656,9 @@ void VolumeMeshNodeT<VolumeMeshT>::draw(GLState& _state, const DrawModes::DrawMo
     // edge based draw mode (except edges on cells) and irregular edges
     edgeDrawMode |= _drawMode & (drawModes_.irregularInnerEdges | drawModes_.irregularOuterEdges);
 
+    // enable twosided lighting for per_face_flat_shaded
+    if(faceDrawMode & (drawModes_.facesColoredPerFaceFlatShaded))
+      _state.set_twosided_lighting(true);
 
     if (cellDrawMode)
         drawCells(_state, cellDrawMode);