Model Viewer fails to load on some older devices

When trying to load model viewer on some old or low end devices (tried an old laptop with intel graphics, a colleague also reported the same behaviour on their 2014 mac, among others) the ARRenderer class fails with an error:

`Uncaught TypeError: this.threeRenderer is undefined`

Which seems to come from line:

`this.threeRenderer.xr.enabled = true;`

Could be related to #4972 as the error looks similar. Nothing displays on the page at all after it fails, and the machines I tested seem able to run threeJS just fine otherwise. I managed to reproduce the error on my setup by using a small win7 container using the dockurr/windows Docker image.