[X11DRV] Re-add support for the 'UseXShm' config key.

[Lionel Ulmer]

> Do you actually need it?  Otherwise I think we might as well remove it
> completely, clearly nobody depends on it since it has been broken for
> a long time.

Well, sometimes, in some rare debugging cases, it's useful to have it, yeah.

Basically, I was hunting down a GFX corruption bug and was wondering if it
was due to the fact that the XShm code path is 'asynchronous' and that we
sent two updates in a row while the first one was not yet finished
processing on the X server side...

And in doing this, I found out that the 'disable XShm' option did not work
anymore and I sent this patch.

          Lionel

-- 
		 Lionel Ulmer - http://www.bbrox.org/