Inform caller of decoder about updated references

Inform the caller of the decoder if a decoded frame updated last,
golden, or altref frames, required for realtime communication
proposed in document VP8 RTP payload format.

Added a new vpx_codec_control called VP8D_GET_LAST_REF_UPDATES, to be
called after vpx_codec_decode. The control will indicate which of the
reference frames that were updated by setting the 3 LSBs in the input
int (pointer).

Change-Id: Iac9db60dac414356c7ffa0b0fede88cb91e11bd7