Move invariant variables to their respective parts of the loop.

[justinmcgrath]

This is mostly done by the optimizer, but it helps a little bit here.

Below is the fastest way, but it requires changing the order of the dimensions, so it would probably break older code.

    for (size_t i = 0; i < last_index; ++i) {
        data[i] = ((double) image[i]) / 255.0;