X11: fix removing SDL_WINDOW_HIDDEN flags (thanks @zturtleman)

src/video/x11/SDL_x11window.c

@@ -306,7 +306,7 @@ static int SetupWindowData(_THIS, SDL_Window *window, Window w, BOOL created)
         window->w = attrib.width;
         window->h = attrib.height;
         if (attrib.map_state != IsUnmapped) {
-            window->flags &= SDL_WINDOW_HIDDEN;
+            window->flags &= ~SDL_WINDOW_HIDDEN;
         } else {
             window->flags |= SDL_WINDOW_HIDDEN;
         }