[gstreamer] Reworking the interface a bit

Remove egl-display property and introduce two new methods. attach_to_display() and detach_from_display()
Some GL implementation like Nexus 4 implementation does not like manipulating the fence objects from
various threads.
We need a mechanism to inform the sink that we are done with rendering so it can destroy any remaining
fence or GL objects from the same rendering thread that created them. This prevents leaking.

While at it, remove egl-display and replace it with a normal function call.