Add support for the X Composite Extension.

For subwindow/appshare modes this allows sharing the respective window/app
when it is partially or completely off-screen as XComposite renders the window
contents to an off-screen buffer. Before, updating of the window contents would
stop once it went even partially off-screen.
Merge pull request #1 from LibVNC/xcomposite

Add support for the X Composite Extension.