Fix ticket #140 inconsistent results when CPR is on

During CPR block vector (BV) search refinement for chroma, BV was converted from integer precision to 1/4-pel precision but motion compensation is with 1/16-pel precision. Consequently, the BV in use is actually half amplitude than it should be, which may make BV point to uncoded region so that different BVs and hence different coding results may be observed for different runs.