XWD option to dump root background

[john at lane.uk.net]

A new -bg option to instruct xwd to dump the root background behind
the selected window (or the whole root background if used with -root).

It allows one to copy the root background (the "desktop wallpaper") as
it currently looks: not the original image that may have been tiled,
centered or stretched but how it actually looks on screen (or screens!)
when copied.

The patch allows a command like this:

    $ xwd -root -bg | convert - bg.png

When used with a non-root window it dumps the area of the root
background that is covered by the selected window (plus its frame if
the -frame option is also used).