Added ability to disable lighting in the viz-module.

When I reconstructed the 3D scene I want to show it as viz::WMesh with an initial value of lighting. For this I disable lighting and shadows:
cv::viz::Viz3d viz_3d_window("3D");
viz_3d_window.setRenderingProperty("mesh", cv::viz::LIGHTING, 0);

Most of the examples show the 3D reconstruction using the point cloud, but using  the mesh one part of it creates a shadow on other parts of it if lighting ON.
Using a 3D mesh with disabled lighting gives the most realistic picture of the reconstructed 3D scene.

modules/viz/include/opencv2/viz/widgets.hpp

@@ -67,7 +67,8 @@ namespace cv
             REPRESENTATION,
             IMMEDIATE_RENDERING,
             SHADING,
-            AMBIENT
+            AMBIENT,
+            LIGHTING
         };
 
         enum RepresentationValues