Clarify unit-length PBR normal/tangent inputs.#2969
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Some normal inputs in tests are not unit length values, despite the code silently assuming them to be unit length.
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When comparing GLSL and OSL on "all tests", I found that GLSL silently assumes in its math that normals are unit length, while OSL renormalizes. The tests contain several non-unit length normals (1,1,1 and 0,0,0 being the most common).
This proposal proposes to follow the existing GLSL implementation and just tighten the spec by mandating that normals, tangents and bitangents are unit length.
If this is rejected, we should explicitly allow them to be non-unit-length and adjust the GLSL implementation to handle the case (e.g., renormalize).