Time to make a mask for the drop shadow.

Take a time out from messing with the workflow window, and create two new
images that we'll use to construct the shadow mask. We only have to do this once,
and then the mask is referenced in the workflow (later), and we never have
to do this sequence of steps again (unless you change the dimensions of the images
you will wrap with drop shadows).

The final image, with drop shadow, will be 780 x 540 pixels,
so make a new 8-bit black image that size.

The picture image (with decorative border) is 744 x 504, so make a new
8-bit white image that size.

We will use the composite transform to combine things in a way that will make the mask.