Add a frame_index entry to RefCntBuffer
Add a frame_index entry to RefCntBuffer

This entry will only be effectively used at the encoder side.
Adding it to the RefCntBuffer data structure would help make the
associated logic a lot simpler. Its effect on the decoder side
would be explicitly sent through the bit-stream.

Change-Id: I1660dce9e0bb6e28c3315d5e0df6dc4a9298f71f

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