Roll libvpx to 4cf4c94ad166.

Relevant updates:

4cf4c94ad166: Fix to rd cost computation for motion vector bias 
(issue: http://code.google.com/p/webm/issues/detail?id=486)

f6886c4b9328: post-proc: fix 0 or negative threshold handling
4c53bacce4a9: post-proc: deblock filter optimization
(issues: http://code.google.com/p/webm/issues/detail?id=480
         http://code.google.com/p/webm/issues/detail?id=479)
Review URL: https://webrtc-codereview.appspot.com/909004

git-svn-id: http://webrtc.googlecode.com/svn/trunk/src@2962 4adac7df-926f-26a2-2b94-8c16560cd09d
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