skip un-neccessary motion search in the first pass

This patch allows the encoder to skip the
un-neccessary motion search in the first pass. It
calculates the error of the zero motion vector using
the last source frame as reference and skips the
further motion search in the first pass if the error
is small.

The encoding speedup of the first pass for slideshow
videos is over 30%. Borg test shows the overall PSNR
performance remain approximately the same (derf -0.009,
hd 0.387, yt 0.021, stdhd 0.065). Individual clips may
have either PSNR gain or loss. The worst PSNR perfomance
is from yt set, with a PSNR loss of -1.1.

Change-Id: I08b2ab110b695e4689573b2567fa531b6457616e