Use slices for image data

[kornelski]

• Makes extend_from_slice manage img_buf's capacity, which avoids reallocations
• safe copy_u8_to_i32 generates the same assembly for the inner loop
• ImageData can actually have a correct lifetime

[jjhbw]

LGTM! Seems like a solid safety and readability improvement.

Did you observe a speedup by chance? I think I see a very small one in detect (<1%). I'm trying to understand criterion a little better, and it seems its not really reliable on my machine. Probably a macOS thing.

Thanks for your great PRs by the way! Out of interest, is there any particular reason that you're hacking on rustface?

[kornelski]

I'm not sure about speed, because I can't set a baseline in criterion. Hey, it's even same issue you've had: bheisler/criterion.rs#193

[jjhbw]

Yeah, I have to use the 'previous benchmark' as a baseline each time... Can get a little annoying as you'll need to run a benchmark of your baseline implementation each time, or manually delete the previous run in /target/criterion.