make bsize requirement for SEG_LVL_SKIP explicit
make bsize requirement for SEG_LVL_SKIP explicit

The segment feature SEG_LVL_SKIP requires the prediction unit size
to be at least BLOCK_8X8. This commit makes the requirement to be
explicit. This is to prevent future encoder implementations from
making wrong choices.

Change-Id: I0127f0bd4c66e130b81f0cb0a8d3dbfe3b2da5c2

shacklettbp/aom,jacklicn/webm.libvpx,mbebenita/aom,kleopatra999/webm.libvpx,luctrudeau/aom,ittiamvpx/libvpx-1,liqianggao/libvpx,hsueceumd/test_hui,thdav/aom,jacklicn/webm.libvpx,lyx2014/libvpx_c,shacklettbp/aom,mwgoldsmith/vpx,gshORTON/webm.libvpx,abwiz0086/webm.libvpx,Laknot/libvpx,GrokImageCompression/aom,mwgoldsmith/libvpx,openpeer/libvpx_new,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,jacklicn/webm.libvpx,gshORTON/webm.libvpx,Acidburn0zzz/webm.libvpx,jdm/libvpx,jmvalin/aom,jdm/libvpx,kim42083/webm.libvpx,running770/libvpx,WebRTC-Labs/libvpx,jmvalin/aom,Suvarna1488/webm.libvpx,luctrudeau/aom,ittiamvpx/libvpx,pcwalton/libvpx,lyx2014/libvpx_c,Maria1099/webm.libvpx,mbebenita/aom,charup/https---github.com-webmproject-libvpx-,mwgoldsmith/libvpx,matanbs/vp982,luctrudeau/aom,running770/libvpx,openpeer/libvpx_new,ittiamvpx/libvpx-1,n4t/libvpx,ittiamvpx/libvpx,matanbs/webm.libvpx,kleopatra999/webm.libvpx,altogother/webm.libvpx,openpeer/libvpx_new,mbebenita/aom,shacklettbp/aom,WebRTC-Labs/libvpx,Topopiccione/libvpx,WebRTC-Labs/libvpx,Maria1099/webm.libvpx,goodleixiao/vpx,webmproject/libvpx,turbulenz/libvpx,stewnorriss/libvpx,abwiz0086/webm.libvpx,Maria1099/webm.libvpx,hsueceumd/test_hui,kalli123/webm.libvpx,sanyaade-teachings/libvpx,felipebetancur/libvpx,liqianggao/libvpx,GrokImageCompression/aom,shareefalis/libvpx,Laknot/libvpx,mbebenita/aom,hsueceumd/test_hui,Distrotech/libvpx,Laknot/libvpx,ShiftMediaProject/libvpx,reimaginemedia/webm.libvpx,kalli123/webm.libvpx,ShiftMediaProject/libvpx,smarter/aom,abwiz0086/webm.libvpx,matanbs/vp982,Topopiccione/libvpx,kleopatra999/webm.libvpx,liqianggao/libvpx,kleopatra999/webm.libvpx,jdm/libvpx,altogother/webm.libvpx,goodleixiao/vpx,Suvarna1488/webm.libvpx,ittiamvpx/libvpx,luctrudeau/aom,mwgoldsmith/vpx,stewnorriss/libvpx,lyx2014/libvpx_c,lyx2014/libvpx_c,cinema6/libvpx,running770/libvpx,openpeer/libvpx_new,jmvalin/aom,Laknot/libvpx,n4t/libvpx,pcwalton/libvpx,thdav/aom,webmproject/libvpx,stewnorriss/libvpx,hsueceumd/test_hui,felipebetancur/libvpx,mwgoldsmith/vpx,Acidburn0zzz/webm.libvpx,Suvarna1488/webm.libvpx,Topopiccione/libvpx,altogother/webm.libvpx,turbulenz/libvpx,mbebenita/aom,sanyaade-teachings/libvpx,abwiz0086/webm.libvpx,goodleixiao/vpx,zofuthan/libvpx,webmproject/libvpx,iniwf/webm.libvpx,n4t/libvpx,ittiamvpx/libvpx-1,mwgoldsmith/libvpx,turbulenz/libvpx,altogother/webm.libvpx,VTCSecureLLC/libvpx,lyx2014/libvpx_c,shyamalschandra/libvpx,webmproject/libvpx,matanbs/webm.libvpx,matanbs/vp982,shyamalschandra/libvpx,mwgoldsmith/vpx,sanyaade-teachings/libvpx,jdm/libvpx,shareefalis/libvpx,kim42083/webm.libvpx,abwiz0086/webm.libvpx,luctrudeau/aom,gshORTON/webm.libvpx,Acidburn0zzz/webm.libvpx,cinema6/libvpx,VTCSecureLLC/libvpx,reimaginemedia/webm.libvpx,Topopiccione/libvpx,shyamalschandra/libvpx,zofuthan/libvpx,altogother/webm.libvpx,webmproject/libvpx,VTCSecureLLC/libvpx,kleopatra999/webm.libvpx,thdav/aom,jmvalin/aom,cinema6/libvpx,shareefalis/libvpx,smarter/aom,kim42083/webm.libvpx,ShiftMediaProject/libvpx,kleopatra999/webm.libvpx,webmproject/libvpx,Distrotech/libvpx,liqianggao/libvpx,stewnorriss/libvpx,matanbs/webm.libvpx,goodleixiao/vpx,n4t/libvpx,ittiamvpx/libvpx-1,ittiamvpx/libvpx-1,mwgoldsmith/libvpx,Suvarna1488/webm.libvpx,felipebetancur/libvpx,kalli123/webm.libvpx,shareefalis/libvpx,Distrotech/libvpx,Acidburn0zzz/webm.libvpx,lyx2014/libvpx_c,abwiz0086/webm.libvpx,Distrotech/libvpx,reimaginemedia/webm.libvpx,liqianggao/libvpx,zofuthan/libvpx,ittiamvpx/libvpx,ittiamvpx/libvpx,gshORTON/webm.libvpx,jmvalin/aom,mwgoldsmith/libvpx,felipebetancur/libvpx,shyamalschandra/libvpx,zofuthan/libvpx,iniwf/webm.libvpx,openpeer/libvpx_new,charup/https---github.com-webmproject-libvpx-,Acidburn0zzz/webm.libvpx,mbebenita/aom,iniwf/webm.libvpx,running770/libvpx,Distrotech/libvpx,zofuthan/libvpx,mbebenita/aom,matanbs/vp982,gshORTON/webm.libvpx,Topopiccione/libvpx,charup/https---github.com-webmproject-libvpx-,cinema6/libvpx,reimaginemedia/webm.libvpx,sanyaade-teachings/libvpx,kalli123/webm.libvpx,ittiamvpx/libvpx-1,thdav/aom,felipebetancur/libvpx,sanyaade-teachings/libvpx,jdm/libvpx,turbulenz/libvpx,Topopiccione/libvpx,thdav/aom,GrokImageCompression/aom,shareefalis/libvpx,reimaginemedia/webm.libvpx,felipebetancur/libvpx,goodleixiao/vpx,mwgoldsmith/vpx,running770/libvpx,jmvalin/aom,charup/https---github.com-webmproject-libvpx-,VTCSecureLLC/libvpx,Suvarna1488/webm.libvpx,kalli123/webm.libvpx,Distrotech/libvpx,ittiamvpx/libvpx,reimaginemedia/webm.libvpx,turbulenz/libvpx,WebRTC-Labs/libvpx,luctrudeau/aom,turbulenz/libvpx,liqianggao/libvpx,matanbs/vp982,matanbs/webm.libvpx,WebRTC-Labs/libvpx,kalli123/webm.libvpx,pcwalton/libvpx,turbulenz/libvpx,matanbs/vp982,thdav/aom,stewnorriss/libvpx,GrokImageCompression/aom,kim42083/webm.libvpx,kim42083/webm.libvpx,VTCSecureLLC/libvpx,openpeer/libvpx_new,mwgoldsmith/vpx,hsueceumd/test_hui,smarter/aom,running770/libvpx,Acidburn0zzz/webm.libvpx,mwgoldsmith/libvpx,shacklettbp/aom,matanbs/webm.libvpx,turbulenz/libvpx,turbulenz/libvpx,charup/https---github.c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