Merge changes Ia82cef79,I7324a75a,I7b66daad,I73344451,I91dc210f,I5945b5ce into experimental

* changes:
  Use alt-ref frame context for keyframes
  Preserve the previous golden frame on golden updates
  Generalize and increase frame coding contexts
  Start to anonymize reference frames
  Update encoder to use fb_idx_ref_cnt
  Remove buffer-to-buffer copy logic