vaapisink: keep handle_events flag except that if user want to set
vaapisink: keep handle_events flag except that if user want to set

When state of vaapisink is changed from PLAYING to NULL, the handle_events
flag is set to FALSE, and never recovered, and then event thread is never
going to run.

So we should allow to set the flag only when users try it.

https://bugzilla.gnome.org/show_bug.cgi?id=782543

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