Enable mixing intra and inter blocks in inter frames (#469)

* WIP motion vector reference search

* More searches

* Have a version that compiles

* Actually call MV prediction function

Fix a few bugs along the way

* Add computation of context for inter mode

* Have find_mvrefs return a mode_context value and use it

* Test but never select intra mode in inter frame

* Fixes to context derivation for inter mode

Some success with mixing intra and inter in a same frame but
still buggy

* Fixes to find MV references functions

Can encode/decode 20 frames at speed 4, but not at slower speed
settings

* Fix MV scan

Now intra/inter mixing works for all speed settings

* Fix issues that arose from rebase

* Remove lines that should not have been added

* Remove superfluous empty lines

* Supress compiler warnings

* A few cosmetic improvements