x11: XI_Motion should mark the global mouse position as changed.

Otherwise we'll miss it when XWarpPointer() is used. x11vnc may do this to
manage mouse input in some circumstances, so it can be possible for _all_
mouse motion to go through this path, breaking SDL_GetGlobalMouseState().

Thanks to @chrismile for all the detective work to figure this out!

Fixes #8827.

src/video/x11/SDL_x11xinput2.c

@@ -337,6 +337,8 @@ int X11_HandleXinput2Event(SDL_VideoData *videodata, XGenericEventCookie *cookie
         const XIDeviceEvent *xev = (const XIDeviceEvent *)cookie->data;
         int pointer_emulated = (xev->flags & XIPointerEmulated);
 
+        videodata->global_mouse_changed = SDL_TRUE;
+
         if (!pointer_emulated) {
             SDL_Mouse *mouse = SDL_GetMouse();
             if (!mouse->relative_mode || mouse->relative_mode_warp) {