In order to apply normal-maps to a mesh correctly, we have to compute the tangent-space vectors (normals, tangents, binormals/-tangents) exactly like the program did that created the normal-map (by projecting the details of a high-poly mesh onto a normal-map for use on the corresponding low-detail mesh).
Different programs may do this differently, e.g. averaging the tangent-space vectors of triangle with equal weight, or weighted by triangle area, or weighted by triangle angle at the common vertex, etc.
Also, we may wish to have a variant where no smoothing (averaging) is computed at all, e.g. for crates and boxes and other very simple models, or for testing.
The same facility would allow us to get rid of the `gts_*` members in `CafuModelT::MeshT`. (If we removed the `gts_*` members, we have to save all currently used `ase` and `lwo` models appropriately (with the proper tangent-space variant set) as `cmdl` files.)