plugins: fix memory leak when GL context is created
plugins: fix memory leak when GL context is created

When the GL display and context are created inside an VAAPI element
the created GL context is leaked.

https://bugzilla.gnome.org/show_bug.cgi?id=773453

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