Eliminated prev_mip memsets/memcpys in encoder
Eliminated prev_mip memsets/memcpys in encoder

This patch is in experimental but was not merged into master.

This patch swaps ptrs instead of copying and uses the
last show_frame flag instead of setting the entire buffer
to zero.

Change-Id: Ia0950466c8ba301a2a5bf917ff3d07bc1a2c2311

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