Add apply_as_mask operation

I had a need for an operator like this when I wanted to apply a circular layer mask to an image. I thought it might be useful for others too!

Starting with an image, you create a canvas of the same size with a white background. Then you 'colour in' bits of the new canvas black. Use `mask.apply_as_image(image)` to modify the mask replacing all black pixels with their respective image pixels.
That's how I started, but since I wanted to round the image to a circle, I wasn't sure how to create a canvas with a background colour of white, then 'cut out' a circle of transparency in the middle.