pickmode: only cost the skip flag once per prediction block
pickmode: only cost the skip flag once per prediction block

RTC speed 6:
File    Match   Avg     BDRate  Low     Mid     High
OVERALL ✔       -0.040  -0.045  -0.031  -0.084  0.004

Screencast speed 6:
File     Match   Avg     BDRate  Low     Mid     High
OVERALL ✔       1.115   -0.162  0.203   2.470   0.541

Change-Id: I46bbc11c89301015b5d3eac25294c709f23f0897

Topopiccione/libvpx,luctrudeau/aom,Topopiccione/libvpx,luctrudeau/aom,mwgoldsmith/libvpx,luctrudeau/aom,mwgoldsmith/libvpx,webmproject/libvpx,webmproject/libvpx,GrokImageCompression/aom,mwgoldsmith/libvpx,smarter/aom,GrokImageCompression/aom,mwgoldsmith/vpx,GrokImageCompression/aom,mwgoldsmith/vpx,smarter/aom,mbebenita/aom,GrokImageCompression/aom,smarter/aom,mbebenita/aom,webmproject/libvpx,luctrudeau/aom,mwgoldsmith/vpx,mbebenita/aom,smarter/aom,mbebenita/aom,mbebenita/aom,GrokImageCompression/aom,smarter/aom,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/vpx,mwgoldsmith/vpx,mwgoldsmith/libvpx,mwgoldsmith/libvpx,Topopiccione/libvpx,mbebenita/aom,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,smarter/aom,mbebenita/aom,Topopiccione/libvpx,Topopiccione/libvpx,Topopiccione/libvpx,webmproject/libvpx,GrokImageCompression/aom,ShiftMediaProject/libvpx,webmproject/libvpx,mwgoldsmith/libvpx,mbebenita/aom,luctrudeau/aom,luctrudeau/aom,mwgoldsmith/vpx,webmproject/libvpx,mbebenita/aom