libvpx

Skip ref frame mode search conditioned on predicted mv residuals

This commit makes the RTC coding mode to conditionally skip the
reference frame mode search, when the predicted motion vector of
the current reference frame gives more than two times sum of
absolute difference compared to that of other reference frames.

It reduces the runtim by 1% - 4% for speed -5 and -6. The average
compression performance is improved by about 0.1% in both settings.

It is of particular benefit to light change scenarios. The
compression performance of test clip mmmovingvga.y4m is improved by
6.39% and 15.69% at high bit rates for speed -5 and -6, respectively.

Speed -5
vidyo1 16555 b/f, 40.818 dB, 12422 ms ->
       16552 b/f, 40.804 dB, 12100 ms

nik    33211 b/f, 39.138 dB, 11341 ms ->
       33228 b/f, 39.139 dB, 11023 ms

mmmoving 33263 b/f, 40.935 dB, 13508 ms ->
         33256 b/f, 41.068 dB, 12861 ms

Speed -6
vidyo1 16541 b/f, 40.227 dB, 8437 ms ->
       16540 b/f, 40.220 dB, 8216 ms

nik    33272 b/f, 38.399 dB, 7610 ms ->
       33267 b/f, 38.414 dB, 7490 ms

mmmoving 33255 b/f, 40.555 dB, 7523 ms ->
         33257 b/f, 40.975 dB, 7493 ms

Change-Id: Id2aef76ef74a3cba5e9a82a83b792144948c6a91