CE3.h: Merge candidates derivation from vector shifting

This proposal is a result of harmonization of techniques in JCT3V-D0178 and JCT3V-D0109. In JCT3V-D0178, up to two candidates may be inserted into a merge candidate list, wherein a candidate is derived by left/right shifting of the disparity vector to identify a candidate in a block of the reference view, if that candidate is unavailable, a disparity motion vector from the spatial neighboring blocks or the disparity vector is left/right shifted to create the candidate. In JCT3V-D0109, a candidate is purely derived by a shift considering the width and height of the current PU. In this joint contribution, it is proposed to add only one additional candidate in the merge candidate list. The additional candidate is firstly generated by shifting the disparity vector by considering the size of the current PU and utilizing the motion information of an identified block in the reference view, as it is done in JCT3V-D0109. Furthermore, similar to JCT3V-D0178, if the block doesn't provide an available candidate, the candidate is derived by shifting horizontally a disparity motion vector. Compared to the current HTM design, the proposed method achieves a compression efficiency gain of 0.2% in terms of BD rate for both BVSP on and off cases, respectively.