Add option to pad an image to a square

[BritishWerewolf]

This PR is stacked on PR 971.
On my fork, this was branched off the other PR, but I couldn't find a way to do this with PRs - sorry 😅

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It's common to want to create a square image, currently we need to do some manual calculations.

With this PR, we can do something like this, with no arguments to just fill the blank area.

const image = await RawImage.fromURL("https://picsum.photos/600/400");
const resized = await image.resize(300, null); // 300x200
const padded = await resized.padToSquare();    // 300x300

Or we can provide a value, and make the image pad to that size.

const image = await RawImage.fromURL("https://picsum.photos/600/400");
const resized = await image.resize(300, null); // 300x200
const padded = await resized.padToSquare(400); // 400x400

Providing a value that is smaller than either the width or height will just return the image unchanged.

const image = await RawImage.fromURL("https://picsum.photos/600/400");
const resized = await image.resize(300, null); // 300x200
const padded = await resized.padToSquare(100); // 300x200

[xenova]

On my fork, this was branched off the other PR, but I couldn't find a way to do this with PRs - sorry 😅

No worries - we will merge that one first 👍

As for this PR, could you also provide a reference to the function that is in another image processing library (e.g., PIL) which is similar? It's useful to align with existing libraries to make sure we don't stray too far away from python standards. Thanks!

[BritishWerewolf]

Actually, I am going to close this PR.
I originally suggested it because I found myself needing it - despite it not being present elsewhere! - however I have not found myself using it since opening.
Thanks.