Improved visual quality of zebra stripes

The stripes were computed based on the normal vector
of the vertex, not on the fragment. This lead to
visual distortions. Now, we use the normal
vector from the fragment shader as offered by the occt api.

TIGLViewer/shaders/PhongShading-v7.6.fs

@@ -180,7 +180,7 @@ vec4 computeLighting (in vec3 theNormal,
     vec3 v = vec3(0., 0., -1.);
 
     // Direction of the view reflected on the surface
-    vec3 vReflect = 2. * (dot(Normal, v)*Normal - v);
+    vec3 vReflect = 2. * (dot(theNormal, v)*theNormal - v);
 
     // normal vector of the light stripe plane
     vec3 lightDir = normalize(vec3(0., 1., 0.));