Trying to use an xlib GXxor

[Alan Corey]

>
>  To get the "blackcolor" over "whitecolor" with GXxor, you should
> draw with "blackcolor^whitecolor".
>  (because the result would  be
> "whitecolor^(blackcolor^whitecolor)".
>
>  In fact, on most displays blackcolor is an integer with the val-
> ue 0, therefore xor with just blackcolor does nothing.
>

Sure enough, blackcolor is 0, thanks.  I did XSetForeground to
whitecolor and it works fine.  Now I can press onward and wow the
world with my super-duper animation. :)

The last time I tried to use xor it was for signing photographs with
an SVG signature, I found a wide white line with a thin black line on
top worked better.  I don't remember what color I was trying to xor on
but the backgrounds varied all over the place.