Add basic support for nonzero motion vectors (#485)

* Add motion vector paramater to inter prediction function

Motion vector is read from added data field in Block structure

* Add reference frame and motion vector fields/parameters

Reference frame and motion vector are added fields to the
RDOOutput structure and added parameters to the block
encoding function

* Fix inter prediction function for nonzero MVs

* Add call to encode new motion vector

* Import default cdfs for MV coding from libaom

* Fix handling of MV precision in MV coding function

* Add coding of DRL mode

* Add motion vector stack

* Add new MV counter to correctly determine context

* Use motion vectors in MV stack for prediction

* Sort MV stack according to weights

Also update weights after near search

* Fix log2() function

* Compute correct context for DRL flag

* Store MVs with 1/8 pel precision and fix MV generation

* Add some basic motion estimation

Motion estimation is full search (+/-16 pel range in 2-pel increments) and
is done independently for each block

* Fix chroma motion compensation for small blocks

Chroma motion compensation sometimes happens on 2x2 basis