Support normal maps when no tangents exist

## 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.