Extend diff MV limit from +/-256 to +/-1024

Extend diff MV limit from +/-256 to +/-1024

Increase the motion search range by 4x. Change MV_CLASS tree of the
entropy coding to allow two additional mv classes to cover the
extended motion vector limit. The codec determines the effective
motion search range conditioned on the actual frame dimension.

It provides coding gains:

stdhd 0.39%
yt    0.56%
hd    0.47%

Major coding performance gains are packed in several sequences with
intense motion activities, e.g., ped_1080p gains 7% at high bit-rates,
and on average 3%.

TODO: Need to further tune the rate control and motion search units.

Change-Id: Ib842540a6796fbee5a797809433ef6a477c6d78d