This experiment implements the idea of the uniform tri-prediction,
which adds a 3rd inter prediction on top of the existing compound
inter bi-prediction. The final predictor for an inter-coded block is
the average of the compound bi-predictor and the third predictor,
which is indentified by a third reference frame and a third motion
vector.

Change-Id: I7ca8435a441960e7d9702fa09614ed7ae9d0ab02