TBN System

[chFleschutz]

Implement a proper TBN system.
For this each vertex has to specify a tangent which can be calculated when the mesh is loaded.
see: https://learnopengl.com/Advanced-Lighting/Normal-Mapping

[chFleschutz]

Currently TBN system uses generated tangents using screen-space derivatives. This can / already has created issues with very small local changes in UV coords.
This should be fixed by adding a vertix attribute for tangents and only falling back to this trick if they are missing.