[tinyobjloader] Fix map_Kd default

If only a diffuse texture is given (map_Kd) tinyobj modulates it to 60%
grey. We prefer 100%.

--- tiny_obj_loader.h
+++ tiny_obj_loader.h
@@ -2297,12 +2297,13 @@ void LoadMtl(std::map<std::string, int> *material_map,
       ParseTextureNameAndOption(&(material.diffuse_texname),
                                 &(material.diffuse_texopt), token);
 
-      // Set a decent diffuse default value if a diffuse texture is specified
-      // without a matching Kd value.
+      // If only a diffuse map has been set, we want the final color to be
+      // fully defined by that specification. So, set the diffuse color to
+      // white.
       if (!has_kd) {
-        material.diffuse[0] = static_cast<real_t>(0.6);
-        material.diffuse[1] = static_cast<real_t>(0.6);
-        material.diffuse[2] = static_cast<real_t>(0.6);
+        material.diffuse[0] = static_cast<real_t>(1);
+        material.diffuse[1] = static_cast<real_t>(1);
+        material.diffuse[2] = static_cast<real_t>(1);
       }
 
       continue;
