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Fix simplex noise gradient lookups and permutation indices - #2003
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f_simplex2d indexed grad2(g + 1, :) with g already in 1..10, reading past the end of the table (gfortran: "Index '11' of dimension 1 of array 'grad2' above upper bound of 10"). f_simplex3d added + 1 to every permutation lookup although p_vec is declared 0:511, which shifted the hash and could index p_vec(512). Both gradient tables were also listed row by row but reshaped in column-major order, so grad3 held duplicate and missing directions instead of the 12 cube-edge vectors. Index the tables directly, drop the extra offsets so the lookups match Gustavson's reference (perm[ii + perm[jj + perm[kk]]] % 12), reshape with order=[2, 1], and replace real(i + j, 8) with real(i + j, wp). Verified against a Python port of the reference algorithm at 20000 random points (max difference 4e-16 in 2D, 1e-16 in 3D) with -fcheck=all clean, and with 2D and 3D pre_process runs using simplex density and velocity perturbation. No test or example in the suite uses simplex perturbation (3D_IGR_33jet is skipped), so no goldens change.
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f_simplex2d indexed grad2(g + 1, :) with g already in 1..10, reading
past the end of the table (gfortran: "Index '11' of dimension 1 of
array 'grad2' above upper bound of 10"). f_simplex3d added + 1 to every
permutation lookup although p_vec is declared 0:511, which shifted the
hash and could index p_vec(512). Both gradient tables were also listed
row by row but reshaped in column-major order, so grad3 held duplicate
and missing directions instead of the 12 cube-edge vectors.
Index the tables directly, drop the extra offsets so the lookups match
Gustavson's reference (perm[ii + perm[jj + perm[kk]]] % 12), reshape
with order=[2, 1], and replace real(i + j, 8) with real(i + j, wp).
Verified against a Python port of the reference algorithm at 20000
random points (max difference 4e-16 in 2D, 1e-16 in 3D) with
-fcheck=all clean, and with 2D and 3D pre_process runs using simplex
density and velocity perturbation. No test or example in the suite
uses simplex perturbation (3D_IGR_33jet is skipped), so no goldens
change.