Implement creating a solid colour rectangle in the image processing library

[DMarby]

Instead of using a source image file, it would be nice if the image processing library could create and return a solid colour rectangle. 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 colour, width and height of the rectangle.

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