Xext: rename two shadowing variables

Xext: rename two shadowing variables

panoramiX.c: In function 'PanoramiXCreateConnectionBlock':
panoramiX.c:599:10: warning: declaration of 'disableBackingStore' shadows a
global declaration [-Wshadow]
In file included from ../include/windowstr.h:60:0,
                 from panoramiX.c:47:
../include/opaque.h:56:52: warning: shadowed declaration is here [-Wshadow]

panoramiX.c: In function 'PanoramiXConsolidate':
panoramiX.c:834:19: warning: declaration of 'pScreen' shadows a previous
local [-Wshadow]
panoramiX.c:813:15: warning: shadowed declaration is here [-Wshadow]

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Mark Kettenis <kettenis@openbsd.org>