[Libvncserver-common] [IDEA] Use XComposite for x11vnc -id subwin mode

[Christian B. <don...@fr...>]

Hi Karl,
Over the last few days I had the idea to make use of the XComposite
extension for x11vnc. Using its redirection feature makes it possible
to still get the contents of windows which are partly off screen which
means in subwin mode the window still gets updated when moved partly
off screen.

Attached patch implements that for -id subwin mode. Feel free to
do whatever you want with it.

Cheers,
   Christian