Store frame motion vector info

Allocate motion vector information for the frame, and store it
when a superblock (64x64) is encoded.

The unit size of the smallest block is 4x4.

A special requirement by the vp9 spec is that sub 8x8 blocks
of a 8x8 block must have the same reference frame.

There is no such requirement for blocks large or equal to 8x8.

Change-Id: Iba17c568c450361e5d059503c6fb7bc458184c31
3 files changed