KHR_materials_transmission: Undefined function χ^- in the definition of V_T

[pierremoreau]

In the KHR_materials_transmission extension, V_T is defined as using the heaviside function χ⁺ in the first term, and the (AFAICT) undefined function χ^- in the second term. Is the second supposed to be χ⁺ like in the first term, and similar to the definition of V in Appendix B of the 2.0 specification?

[proog128]

Looks like the simplification is wrong, we need the same terms as in G_T instead. I created a pull request that fixes the issue and adds a bit of explanation. I hope it's correct now.