Bug fix to Gabor noise -- we inadvertently used undefined order of evaluation.
Bug fix to Gabor noise -- we inadvertently used undefined order of evaluation.

C and C++ do not specify the order in which function parameters are evaluated.
So in our Gabor implementation, we had this:
        Vec3 x_i_c (rng(), rng(), rng());
The rng() is stateful.  gcc evaluated them right-to-left, but we discovered
the problem when compiling with clang, which evaluates left-to-right, thus
giving us a totally different noise seed!

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