x11: Set the window position when entering fullscreen

Set the window position when entering fullscreen as both real Xorg and XWayland need this in order for the window to be on the correct display.

src/video/x11/SDL_x11window.c

@@ -1348,6 +1348,11 @@ static void X11_SetWindowFullscreenViaWM(_THIS, SDL_Window *window, SDL_VideoDis
         X11_XSendEvent(display, RootWindow(display, displaydata->screen), 0,
                        SubstructureNotifyMask | SubstructureRedirectMask, &e);
 
+        /* Set the position so the window will be on the target display */
+        if (fullscreen) {
+            X11_XMoveWindow(display, data->xwindow, displaydata->x, displaydata->y);
+        }
+
         /* Fullscreen windows sometimes end up being marked maximized by
             window managers. Force it back to how we expect it to be. */
         if (!fullscreen) {