Fix "unresponsible application" issues in Wayland

Polling without wl_display_flush() never responds to ping requests.
In that case ping-pong works only on other events, such as user input
or on frame swapped.

From https://git.merproject.org/mer-core/libsdl/merge_requests/3
Original author: Alexander Akulich <a.akulich@omprussia.ru>

src/video/wayland/SDL_waylandevents.c

@@ -178,6 +178,8 @@ Wayland_PumpEvents(_THIS)
 {
     SDL_VideoData *d = _this->driverdata;
 
+    WAYLAND_wl_display_flush(d->display);
+
     if (SDL_IOReady(WAYLAND_wl_display_get_fd(d->display), SDL_FALSE, 0)) {
         WAYLAND_wl_display_dispatch(d->display);
     }