This changes the behavior of the ext-inter compound modes
as follows:
* NEAREST_NEARESTMV, NEAR_NEARMV, NEW_NEWMV, ZERO_ZEROMV
  work the same as the corresponding compound modes without
  ext-inter
* NEAREST_NEWMV and NEW_NEARESTMV keep their current behavior,
  which is to use the first ref-mv candidate pair for their
  particular reference frames
* NEAR_NEWMV and NEW_NEARMV select between the second, third,
  and fourth candidate pairs.
* NEAREST_NEARMV and NEAR_NEARESTMV use the first candidate
  pair for the NEAREST component and the second, third, or
  fourth candidate pair for the NEAR component.

Also do some minor refactoring and fix compile errors
with --enable-ext-inter --disable-ref-mv

Change-Id: If0d2a60559083b7c1189909d426b705af42d2d0e