yuv: skip frames one at a time to prevent offset overflow

--HG--
branch : stable
slicetype: fix cuTree mv indexing (bug found by herman.chen@rock-chips.com)

This drops the bitrate almost 20% and the SSIM from 0.4-0.9 dB.  I believe this
needs rebalancing.

--HG--
branch : stable
extra : amend_source : c0911b6ac75271a175a4d25b21fac9b6296bbf1a