GLX: allow skipping of visual init wrapping

* GL/glx/glxvisuals.c: added boolean to disable
	  calling init_visuals(). This gives a chance to Xephyr
	  to take over visuals manipulation and avoid a crash at
	  server shutdown in __glXMesaScreenDestroy(), due to the fact
	  that mesa might sees more visual than what it has actually created in
	  init_visuals(). It might see more visuals because Xephyr can augment
	  the number of visuals, dynamically.
	* os/utils.c: the boolean is actually defined here.

os/utils.c

@@ -160,6 +160,7 @@ _X_EXPORT Bool noFontCacheExtension = FALSE;
 #endif
 #ifdef GLXEXT
 _X_EXPORT Bool noGlxExtension = FALSE;
+_X_EXPORT Bool noGlxVisualInit = FALSE;
 #endif
 #ifdef SCREENSAVER
 _X_EXPORT Bool noScreenSaverExtension = FALSE;