CalcTangents: zero vector is invalid for tangent/bitangent

[JensEhrhardt-eOPUS]

This fix helps when the calculated tangent of a face is a zero vector

[turol]

Comparing floats with == is dangerous. What happens if the values are negative zeroes instead of positive zeroes? What if they're sufficiently small (around FLT_MIN) that calculating the length still ends up zero?

[JensEhrhardt-eOPUS]

committed a version that also checks for small vectors

[turol]

0.5 is way too big for this use case. Tangents are supposed to be unit-length.

[JensEhrhardt-eOPUS]

length of (0.5, 0.5, 0.5) == 0.866 is smaller than any normalized 3D vector

[JulianKnodt]

er sorry why not just check the length is sufficiently different from 1? i.e. Length(Vector) - 1 < EPSILON, where EPSILON is a small number such as 1e-3? It's a bit hard to understand the intent from reading the current code

The current implementation marks all vectors invalid if they're in the range [-0.5, 0.5], but I'm not sure if it's problematic if the vector is too large.