Implement coloured overlays in the image processing library

[DMarby]

In order to support future features it would be great to be able to draw semi-transparent overlays over parts or the entirety of an image. This would need to be implemented in our libvips binding, as well as the higher level image processing library.

Ideally, we'd be able to configure the size, location, colour and transparency of the overlay.

https://github.com/DMarby/picsum-photos/tree/main/internal/vips https://github.com/DMarby/picsum-photos/tree/main/internal/image