Support normal maps when no tangents exist

[janhohenheim]

What problem does this solve or what need does it fill?

When adding a material with a normal map to a mesh without tangents, Bevy will silently ignore the normal map

What solution would you like?

Per @DGriffin91

It would be nice to be able to fallback to screen space generated tangets here in the fragment shader, I think the code for this already exists on the meshlet path but could be used here too.

What alternative(s) have you considered?

Warn the user least in this scenario

Additional context

I ran into this and debugging took quite a while, as I expected my normal maps to be the problem. Nope, the tangents on the mesh were missing.