Line accuracy issue in pathgroup with high zoom level

Fix is simple, needs to be evaluated by kangax.
@kangax What if `fabric.Line` is subclassed? Maybe a small helper to check if the object is an instance of fabric.Line?
```js
fabric.isInstanceOf = function (object, instance) {
  return object instanceof instance || object.prototype instanceof instance;
}
```

```js
if (((this.width === 0 || this.height === 0) && !fabric.isInstanceOf(this, fabric.Line)) || !this.visible) {
```