VideoCommon: Add comment explaining why only the first normal gets normalized

Co-authored-by: Scott Mansell <[email protected]>

Source/Core/VideoBackends/Software/TransformUnit.cpp

@@ -97,6 +97,11 @@ void TransformNormal(const InputVertexData* src, OutputVertexData* dst)
   MultiplyVec3Mat33(src->normal[0], mat, dst->normal[0]);
   MultiplyVec3Mat33(src->normal[1], mat, dst->normal[1]);
   MultiplyVec3Mat33(src->normal[2], mat, dst->normal[2]);
+  // The scale of the transform matrix is used to control the size of the emboss map effect, by
+  // changing the scale of the transformed binormals (which only get used by emboss map texgens).
+  // By normalising the first transformed normal (which is used by lighting calculations and needs
+  // to be unit length), the same transform matrix can do double duty, scaling for emboss mapping,
+  // and not scaling for lighting.
   dst->normal[0].Normalize();
 }