CE5.h: Merge candidates derivation from vector shifting

When inter-view motion prediction is enabled, the current HTM design of the merge candidate list may include an inter-view predicted motion candidate before all the spatial merging candidates. In addition, the disparity vector may also be converted to a disparity motion vector and it is inserted into the list following the above-right spatial candidate. As a harmonized follow-up proposal of JCT3V-C0045 and JCT3V-C0148, it is proposed to add two more candidates. Each candidate is derived with a horizontally shifted disparity vector being used for inter-view motion prediction and if it doesn't provide an available candidate, then it is derived from a horizontally shifted spatial neighbouring candidate, if it contains a disparity motion vector. Compared to the current HTM design, the proposed method achieves compression efficiency gain of 0.2% in terms of BD rate for both BVSP on and off cases.