xcb: Check for SHAPE extension before using it

xcb: Check for SHAPE extension before using it

This change makes sure that the X11 server really supports the SHAPE
extension before it is used.

Before this, libxcb would disconnect from the server as soon as it was
told to send a SHAPE request to a server which doesn't support SHAPE. Oh
and of course it would do so without any good error message.

Change-Id: I5f9481e488925e74e62f229e9f637ef262020c04
Signed-off-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>