While doing the mode-1 referece picture selection,
the circular buffer logic was not correctly setting the
refresh frame flags as per VP9 spec.
Make sure refresh_flag[0] get updated correclty after
each cycle of GST_VP9_REF_FRAMES.
https://bugzilla.gnome.org/show_bug.cgi?id=771507
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