glwindow: Make interaction with GstContextEGL more robust
As of current implementation, the native surface is managed by the Window and the EGLWindowSurface is handled by the GstGLContextEGL object. Though, whenever one recreate it's surface the other must do the same, since this is two representation of the same object. It is allowed for one to outlive the other (though this was broken in gallium till now), but then any operation will fail.
@ystreet suggested to improve robustness by making it explicit through a signal from the window to the GstGLContextEGL. This would avoid comparing window handles, which may fail if the handles are reused even though the window was destroyed and re-recreated.