Remove any pointer usage in the code

[keryell]

There are a few superfluous .data() and get_pointer() instead of just using the accessor.

For example instead of

render<width, height, samples>(myQueue, fb.data(), hittables.data(),
                                 hittables.size(), cam);
save_image<width, height>(fb.data());

render<width, height, samples>(myQueue, fb, hittables, cam);
save_image<width, height>(fb);

Also replace hittables_ptr, num_hittables by hittables_acc since the accessor has the size too.

Then probably hit_world and get_color can be just plain functions.
I have the feeling that you could replace

    auto render_kernel = pixel_renderer<width, height, samples, depth>(
        frame_ptr, hittables_ptr, num_hittables, cam);
    executor<width, height>(cgh, render_kernel);

by

    executor<width, height>(cgh, [=] (...) { ... here is the full renderer code });

and in the executor do not copy the lambda:

void executor(auto& cgh, T& render_kernel) {

(or perhaps even const T& render_kernel).

I think all these copies explain the slowdown from #33

Be sure you do not copy anything in a useless way. Pass by reference or at least std::forward.