Draw image rectangles into each other, with alpha blending

Hello!

For a project I'm working on, I needed a nice way of drawing images into other images. After doing the base case, I realised it wasn't going to handle alpha channels, so I've implemented something to solve that too.

Low-level image manipulation isn't my strongest skill, so it might be a bit naive, but it works as a basic implementation.

Let me know if you need any changes.
This function probably belongs in the `imageops` module, maybe as an `overlay` function. Maybe consider supporting different blend modes, like add, multiply, subtract, overlay, etc.?