x11: Mark backing_store as NotUseful when creating windows.

This can give some performance boost, and save some resources, as there's no
reason to keep a copy of an SDL window's contents on the server: most SDL
apps are redrawing completely every frame, and the API allows for expose
events to tell an app a redraw is needed anyhow.

(And compositors are free to ignore this setting if it makes sense to do so,
according to the Xlib docs.)

Reference Issue #3776.

src/video/x11/SDL_x11window.c

@@ -445,6 +445,7 @@ X11_CreateWindow(_THIS, SDL_Window * window)
     }
 
     xattr.override_redirect = ((window->flags & SDL_WINDOW_TOOLTIP) || (window->flags & SDL_WINDOW_POPUP_MENU)) ? True : False;
+    xattr.backing_store = NotUseful;
     xattr.background_pixmap = None;
     xattr.border_pixel = 0;
 
@@ -531,7 +532,7 @@ X11_CreateWindow(_THIS, SDL_Window * window)
                       window->x, window->y, window->w, window->h,
                       0, depth, InputOutput, visual,
                       (CWOverrideRedirect | CWBackPixmap | CWBorderPixel |
-                       CWColormap), &xattr);
+                       CWBackingStore | CWColormap), &xattr);
     if (!w) {
         return SDL_SetError("Couldn't create window");
     }