libs: windows: wayland: fix leak if failure of sync
libs: windows: wayland: fix leak if failure of sync

Sometimes gst_vaapi_window_wayland_sync returns FALSE when poll returns EBUSY
during destruction.
In this case, if GstVaapiWindow is using vpp, leak of vpp surface happens.
This surface is not attached to anything at this moment, so we should release
it manually.

https://bugzilla.gnome.org/show_bug.cgi?id=781695

ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi