Use material-utils where possible

[takahirox]

There some codes in src like

if (object.material) {
  if (Array.isArray(object.material)) {
    object.material.forEach(material => func(material));
  } else {
    func(object.material);
  }
}

utils/material-utils has forEachMaterial helper function that does the above. Other files can just import and use it.

[netpro2k]

LGTM.

Related. I suspect as we convert to typescript we will find more spots where we are assuming material is a single object but can be an array.

[takahirox]

Related. I suspect as we convert to typescript we will find more spots where we are assuming material is a single object but can be an array.

Perhaps right. Let's digging into in another PR (at some point).