Set diffuse to black when constant lighting is used

lib/gltf.js

@@ -90,6 +90,7 @@ function createGltf(objData) {
         if (!hasNormals) {
             // Constant technique only factors in ambient and emission sources - set emission to diffuse
             emission = diffuse;
+            diffuse = [0, 0, 0, 1];
         }
 
         var technique = hasNormals ? (hasSpecular ? 'PHONG' : 'LAMBERT') : 'CONSTANT';