Why crop need a mutable reference to an image

[dfeyer]

The current signature is:

pub fn crop(
    photon_image: &mut [PhotonImage](https://docs.rs/photon-rs/latest/photon_rs/struct.PhotonImage.html),
    x1: [u32](https://doc.rust-lang.org/nightly/std/primitive.u32.html),
    y1: [u32](https://doc.rust-lang.org/nightly/std/primitive.u32.html),
    x2: [u32](https://doc.rust-lang.org/nightly/std/primitive.u32.html),
    y2: [u32](https://doc.rust-lang.org/nightly/std/primitive.u32.html)
) -> [PhotonImage](https://docs.rs/photon-rs/latest/photon_rs/struct.PhotonImage.html)

But inside the implementation, I don't understand why we need a mutable reference. I need to crop really big image, and can be nice the have async cropper, but with the mutability it's a bit messy.

Thanks for the clarification, and thanks for all the hard on it, amazing project