Enable to assign distance based weight for joint compound prediction.

(w0, w1) are weights for two predictors of different distance to
current frame.

Use 4 bit precision for quantized distance weight. e.g.
the prediction is generated as

value = (w0 * p0 + w1 * p1) >> n
w0 + w1 = (1 << n), n = 4;

Change-Id: Ib0ff0c41c82b9ebb033f498e90c18a03d18969e4