Add support for a separate color / image for padding area background

[Psychlist1972]

Summary of the new feature/enhancement

I'd like to be able to specify a separate background color, image, etc. for the area in the margin (padding) in the terminal window.

This uses a background image and padding value, but fails when resized to anything but the golden value of both.

In straight-up XAML, I'd do this by having a window-wide background color/image, and then specifying something different for the area inside the padding.