This commit allows an adaptive motion vector referencing mode
approach. It checks the available reference motion vector candidate
list and decides the amount of motion vector referencing modes. The
current implementation assumes simple binary coding for the syntax.

The compression performance is improved by
derf   0.11%
hevcmr 0.38%
stdhd  0.09%
hevchr 0.23%

The coding gains due to the new reference motion vector system are
derf   1.0%
hevcmr 1.7%
stdhd  1.4%
hevchr 1.3%

Change-Id: Idf932fc373546fe59c8741f1b933ff656e8dbc3f