Add motion search skipping in first pass

This change added a motion search skipping mechanism similar
to what we did in second pass. For a macroblock that is very
similar to the macroblock at same location on last frame,
we can set its mv to be zero, and skip motion search. This
improves first-pass performance for slide shows and video
conferencing clips with a slight PSNR loss.

Change-Id: Ic73f9ef5604270ddd6d433170091d20361dfe229