x11: Ensure that parent windows for a dialog are from the X11 driver

If the system doesn't have Zenity installed, message boxes can fall back to the X11 toolkit while the Wayland driver is in use, in which case, a Wayland window may be set as a parent to an X11 dialog. Ensure that the parent window for an X11 dialog is actually an X11 window.

src/video/x11/SDL_x11messagebox.c

@@ -288,6 +288,8 @@ static void X11_OnMessageBoxScaleChange(SDL_ToolkitWindowX11 *window, void *data
 
 static bool X11_ShowMessageBoxImpl(const SDL_MessageBoxData *messageboxdata, int *buttonID)
 {
+    SDL_VideoDevice *video = SDL_GetVideoDevice();
+    SDL_Window *parent_window = NULL;
     SDL_MessageBoxControlsX11 controls;
     SDL_MessageBoxCallbackDataX11 data;
     const SDL_MessageBoxColor *colorhints;
@@ -305,10 +307,14 @@ static bool X11_ShowMessageBoxImpl(const SDL_MessageBoxData *messageboxdata, int
     }
 
     /* Create window */
+    if (messageboxdata->window && SDL_strcmp(video->name, "x11") == 0) {
+        // Only use the window as a parent if it is from the X11 driver.
+        parent_window = messageboxdata->window;
+    }
 #if SDL_FORK_MESSAGEBOX
-    controls.window = X11Toolkit_CreateWindowStruct(messageboxdata->window, NULL, SDL_TOOLKIT_WINDOW_MODE_X11_DIALOG, colorhints, true);
+    controls.window = X11Toolkit_CreateWindowStruct(parent_window, NULL, SDL_TOOLKIT_WINDOW_MODE_X11_DIALOG, colorhints, true);
 #else 
-    controls.window = X11Toolkit_CreateWindowStruct(messageboxdata->window, NULL, SDL_TOOLKIT_WINDOW_MODE_X11_DIALOG, colorhints, false);
+    controls.window = X11Toolkit_CreateWindowStruct(parent_window, NULL, SDL_TOOLKIT_WINDOW_MODE_X11_DIALOG, colorhints, false);
 #endif
     controls.window->cb_data = &controls;
     controls.window->cb_on_scale_change = X11_OnMessageBoxScaleChange;