Fix T38439: allow IOR in range [0, inf] instead of [1, inf] in Cycles.

The same can be achieved by flipping normals on the mesh, but it can be
convenient to do this in the shader.

intern/cycles/kernel/shaders/node_fresnel.osl

@@ -22,7 +22,7 @@ shader node_fresnel(
 	normal Normal = N,
 	output float Fac = 0.0)
 {
-	float f = max(IOR, 1.0 + 1e-5);
+	float f = max(IOR, 1e-5);
 	float eta = backfacing() ? 1.0 / f: f;
 	float cosi = dot(I, Normal);
 	Fac = fresnel_dielectric_cos(cosi, eta);