Changing canvas size

[Andy-92]

Hi,

I am in the position where I need to be able to change the size of the
canvas an image is in (WITHOUT changing the original image, i.e. add a 50
pixel border around the image, so the end dimensions of the image are 50
pixels greater, just filled with the background color)

I can't think of any easy way to do this, as the resize command is more of a
scale command, which is not what I need.

Thanks in advance for any help!
--Andrew