[Rendering] Support loading tangents from assets again

[kanerogers]

Background

We've gone back and forth on tangents a couple times; @rasmusgo found that computing tangents on the fly is actually quicker, so we've implemented this in https://github.com/leetvr/hotham/pull/342

However, some models simply require tangents that were calculated offline, eg.

[kanerogers]

False alarm! This can be fixed by baking a tangent space normal map in substance painter:

[rasmusgo]

Geez, you startled me! 😅