Do not harness backing store by default

xorg-server 1.15 enables backing store if composite extension is enabled
(default settings). Harnessing backing store through compositor leads to
tearing effect.

This patch reverts default harnessing backing store to conditional use if
SDL_VIDEO_X11_BACKINGSTORE environment variable exists.

<https://bugzilla.libsdl.org/show_bug.cgi?id=2383>
<https://bugzilla.redhat.com/show_bug.cgi?id=1073057>

diff -up SDL-1.2.15/src/video/x11/SDL_x11video.c.jx SDL-1.2.15/src/video/x11/SDL_x11video.c
--- SDL-1.2.15/src/video/x11/SDL_x11video.c.jx	2012-01-19 01:30:06.000000000 -0500
+++ SDL-1.2.15/src/video/x11/SDL_x11video.c	2014-03-04 14:39:34.691545549 -0500
@@ -1088,7 +1088,7 @@ static int X11_CreateWindow(_THIS, SDL_S
 		}
 	}
 
-#if 0 /* This is an experiment - are the graphics faster now? - nope. */
+#if 1 /* This is an experiment - are the graphics faster now? - nope. */
 	if ( SDL_getenv("SDL_VIDEO_X11_BACKINGSTORE") )
 #endif
 	/* Cache the window in the server, when possible */