If we are the clipboard owner when we are shutdown, we recieve a
WM_RENDERALLFORMATS, to render the clipboard, so it's contents will remain
available to other applications.  Unfortunately, this is far too late to do
anything useful with, as the server is waiting for the clipboard thread to exit,
and so can't process requests to convert clipboard contents.

Change so we just do nothing on WM_RENDERALLFORMATS. (I'm not convinced that
WM_RENDERALLFORMATS has ever worked usefully, in any case).

(To make this work, I guess we would need to rearrange the way shutdown works
completely: first synchronously stop the clipboard, then stop the X server)

We also then receive a WM_DRAWCLIPBOARD, perhaps telling us that the available
clipboard formats have changed (as ones which haven't been rendered are now
removed), but the clipboard owner is now the system, not us, which we have to
arrange to ignore.

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Colin Harrison <colin.harrison@virgin.net>