x11: Treat WM setting the window "fullscreen" like FULLSCREEN_DESKTOP.

Fixes #5390.

src/video/x11/SDL_x11window.c

@@ -222,7 +222,7 @@ X11_GetNetWMState(_THIS, Window xwindow)
         }
 
         if (fullscreen == 1) {
-            flags |= SDL_WINDOW_FULLSCREEN;  /* !!! FIXME: arguably this is more like FULLSCREEN_DESKTOP...? */
+            flags |= SDL_WINDOW_FULLSCREEN_DESKTOP;
         }
 
         /* If the window is unmapped, numItems will be zero and _NET_WM_STATE_HIDDEN