Errors if window width is less than background width.

If the width of the Emacs frame is less than the width of the background image, one soft error and one hard error will occur:

- The background overwrites the buffer's newlines, widening the buffer in the background lines.
- Toward the end of the background write it will eventually attempt to `setf` past `(point-max)`, throwing an error.

I would suggest restricting the background line width (e.g. via the `setf` range) to the lesser of the background image line length or the buffer width (using the same logic for the buffer width as elsewhere in snow.el).