Report the required threshold

[emilk]

I would like it if dify::diff::get_results could return what threshold I would need to set in order to get the test to pass.
It should probably return some struct with that, the number of failed pixels, and the diff-image, e.g.:

pub struct DiffOutput {
    pub num_failed_pixels: usize,

    /// The largest pixel-error.
    /// If you set the threshold to this (or higher),
    /// the test will always pass.
    pub max_error: f32,

    pub diff_image: RgbaImage,
}