Incorporate OrderHint concept to motion field init
Incorporate OrderHint concept to motion field init

This CL has done a revisit on the motion field initial setup and
completed a code refactoring. It does not change any of the
functionalies:

* The value of 'cur_frame_offset' associated with each buffered ref
  is indeed the OrderHint in the spec;

* Not all the buffered buf_idx / order_hint are being used in the
  motion field setup, but the use of arrays seems help the code more
  organized.

Change-Id: Ie18d95aadb37c39ff6808dc84ebd5ac7d1dc2618

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom