(1) This patch will allow compound predictions from the same 
    reference;

(2) The code from this patch only takes effect when the sign bias
    values of the two references are allowed to be identical;

(3) A further effort will be taken to combine this effort with
    compound-singleref to finalize a new coding tool for the 
    single reference, compound prediction mode.

Change-Id: I682d9e00c849b02dff76577b2c581e1c1b97770b