Use zero MV for all blocks in inter frames (#408)

* Create proper structure for reference frame buffer

Also move reference frame buffer into frame invariants to
avoid having to pass an additional parameter to each function
requiring access to reference frame buffer.

* Add basic inter prediction function

Basic inter prediction function copies collocated samples
from first frame in reference frame set.

* Add mapping from reference frame index to slot in buffer

Add array that maps a reference frame index such as LAST_FRAME
into a slot of the reference frame buffer

* Use GLOBALMV instead of ZEROMV

For consistency with text specification

* Add support for writing reference frames in inter mode

* Add write_inter_mode() for NEWMV and GLOBALMV inter pred modes

Also,
- Rename ZEROMV prediction mode to GLOBALMV
- RDO search include GLOBALMV mode if inter frame
- Import CDFs required to encode NEWMV or GLOBALMV modes
- Add write_tx_tree() for tx block coding for a inter mode. This
function is also for var-tx, but currently only one tx block for a inter
mode partition since tx_mode in frame header is set as LARGEST_TX atm.

* Fix ref_frame storage and counting

* Add basic inter prediction function

Basic inter prediction function copies collocated samples
from first frame in reference frame set.

* Add mapping from reference frame index to slot in buffer

Add array that maps a reference frame index such as LAST_FRAME
into a slot of the reference frame buffer

* Use GLOBALMV instead of ZEROMV

For consistency with text specification

* Fix entropy coding issues for inter modes