Fix desync introduced by dd3fca176f087d8fe72ddf2e972d728ef92d941f

Note that dd3fca176f087d8fe72ddf2e972d728ef92d941f itself fixed by
accident another desync introduced by
f317efd3bd8fa07ba5b7cfca139712ac3b200a78.
Add more assertions to level 3 MV encoding

This should make it harder to introduce desyncs like the one fixed by
by the previous commit.
Fix exp
simpler mv adaptation