Eevee: Volumetrics: Do not add anisotropy attribute, average it.

This makes no sense to add theses. In cycles, each volume node is computed separatly. In eevee only the combined parameters are evaluated and phase should be averaged in this case.

source/blender/draw/engines/eevee/shaders/bsdf_common_lib.glsl

@@ -46,7 +46,7 @@ Closure closure_add(Closure cl1, Closure cl2)
 	cl.absorption = cl1.absorption + cl2.absorption;
 	cl.scatter = cl1.scatter + cl2.scatter;
 	cl.emission = cl1.emission + cl2.emission;
-	cl.anisotropy = cl1.anisotropy + cl2.anisotropy;
+	cl.anisotropy = (cl1.anisotropy + cl2.anisotropy) / 2.0; /* Average phase (no multi lobe) */
 	return cl;
 }