Add test for NEARMV mode (#523)

* Correctly handle NEAR mode when less than 2 entried in MV stack

If there are fewer than 2 entries in the motion vector stack, then
the NEAR motion vector is considered to be the same vector as the
global motion vector (currently 0,0), as per 7.10.2.12
"Extra search process" in the spec.

* Add support for 2nd and 3rd NEAR motion vector

* Adaptively select best MV mode