Replace all gstvaapiobject in vaapi encoders with standard gstobject.
Let the gstobject common logic to handle all the init and finalize
works. But the property install/set/get still use the old way, need
to be improved later.
Added two modes(as properties) for reference picture selection:
ref-mode-0: AltRef and GoldRef pointing to the recent keyframe
and LastRef is pointing to the previous frame.
ref-mode-1: Previous frame (n) as LastRef , n-1 th frame as GoldRef
and n-2 th frame as AltRef
https://bugzilla.gnome.org/show_bug.cgi?id=766048
