Comment 16 for bug 169623

Here is a different approach to the problem, inspired by the comment from Equis. Rather than add a new control for window color, this patch simply sets the window color to the drawing background color, ignoring alpha. In other words, setting the drawing's background color to #00000000 (fully transparent black) will cause the drawing window to have a black background rather than white as before. Export is not affected, however, so the final .png file will still be fully transparent. If someone could review and commit this, that would be great!