Add pixel_threshold and improve threshold docs

[lucasmerlin]

Sometimes visual threshold isn't enough and it would be nice to just allow x pixels to be different

[teh-cmc]

Workaround we use right now:

    let broken_percent_threshold = 0.003;
    let num_pixels = (size.x * size.y).ceil() as f64;

    use re_viewer_context::external::egui_kittest;
    match harness.try_snapshot(name) {
        Ok(_) => {}

        Err(err) => match err {
            egui_kittest::SnapshotError::Diff {
                name,
                diff: num_broken_pixels,
                diff_path,
            } => {
                let broken_percent = num_broken_pixels as f64 / num_pixels;
                re_log::debug!(num_pixels, num_broken_pixels, broken_percent);
                assert!(
                    broken_percent <= broken_percent_threshold,
                    "{name} failed because {broken_percent} > {broken_percent_threshold}\n{diff_path:?}"
                )
            }

            _ => panic!("{name} failed: {err}"),
        },
    }

[emilk]

@Wumpf Do we still need this after mergning

• Disable anti-aliasing in tests rerun-io/rerun#8971 ?

[Wumpf]

Yes absolutely. Even with AA disabled, we still have one (down from several) test where this is needed and there is many reasons why we may run into problems that need this.