skip the un-necessary motion search in the first pass

This patch allows the VP9 encoder to skip the un-necessary
motion search in the first pass. It computes the motion error
of 0,0 motion using the last source frame as the reference,
and skips the further motion search if this error is small.

Borg test shows overall the patch gives PSNR gain (derf -0.001%,
yt 0.341%, hd 0.282%). Individual clips may have PSNR gain or
loss. The best PSNR performance is 7.347% and the worst is -0.662%.
The first pass encoding speedup for slideshow clips is over 30%.

Change-Id: I4cac4dbd911f277ee858e161f3ca652c771344fe