Do not load prepass normals for transmissive materials

[coreh]

Turns out whenever a normal prepass was active (which includes whenever you use SSAO) we were attempting to read the normals from the prepass for the specular transmissive material. Since transmissive materials don't participate in the prepass (unlike opaque materials) we were reading the normals from “behind” the mesh, producing really weird visual results.

Objective

• Fixes SSAO Breaks Transmission #11112.

Solution

• We introduce a new READS_VIEW_TRANSMISSION_TEXTURE mesh pipeline key;
• We set it whenever the material properties has the reads_view_transmission_texture flag set; (i.e. the material is transmissive)
• If this key is set we prevent the reading of normals from the prepass, by not setting the LOAD_PREPASS_NORMALS shader def.

---

Changelog

Fixed

• Specular transmissive materials no longer attempt to erroneously load prepass normals, and now work correctly even with the normal prepass active (e.g. when using SSAO)