secondlife/viewer#1847: Add test plan for normal textures on PBR materials

doc/testplans/pbr_materials.md

@@ -0,0 +1,12 @@
+# PBR Materials
+
+## KHR Texture Transforms
+
+Texture repeats for PBR materials on prims are based on the [KHR\_texture\_transform](https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_texture_transform) spec, and thus should be expected to behave according to the spec.
+
+PBR materials should have approximately correct lighting based on the normal texture:
+
+- With default texture transforms, assuming the prim or model has correct normals and tangents
+- With a texture transform applied, especially rotation or negative scale
+- With a texture animation applied via `llSetTextureAnim`, especially a rotation animation
+    - Note: Texture animations are not guaranteed to loop when a PBR texture transform is applied