Add OpenMP

I think it should be possible to rewrite the loops to include OpenMP pragmas. The final update could be protected with #pragma omp atomic and you can use default(shared) private(tx ty ix iy) firstprivate(dataptr) or something similar. You'll need to redo the loops, however, you'll need for loops instead of the inner while (or maybe the outer do while), and the stride part might need to be precalculated in some way.