vp8: modifcatiion to skin map computation.
vp8: modifcatiion to skin map computation.

For each block in pickinter: use average of four middle
pixels (instead of single pixel) to set skin map.
This can help a little in reducing false skin detection in
some cases.

Change-Id: Ic247af75e9c2948b08ab977a39e061adacd8ec97

GrokImageCompression/aom,zofuthan/libvpx,ittiamvpx/libvpx-1,zofuthan/libvpx,luctrudeau/aom,ittiamvpx/libvpx-1,felipebetancur/libvpx,thdav/aom,openpeer/libvpx_new,webmproject/libvpx,mbebenita/aom,ShiftMediaProject/libvpx,mbebenita/aom,thdav/aom,openpeer/libvpx_new,felipebetancur/libvpx,GrokImageCompression/aom,thdav/aom,ShiftMediaProject/libvpx,GrokImageCompression/aom,liqianggao/libvpx,smarter/aom,mwgoldsmith/libvpx,zofuthan/libvpx,thdav/aom,VTCSecureLLC/libvpx,jmvalin/aom,mbebenita/aom,jmvalin/aom,Topopiccione/libvpx,smarter/aom,openpeer/libvpx_new,mwgoldsmith/vpx,mwgoldsmith/vpx,liqianggao/libvpx,liqianggao/libvpx,Topopiccione/libvpx,jmvalin/aom,Topopiccione/libvpx,liqianggao/libvpx,smarter/aom,mwgoldsmith/vpx,zofuthan/libvpx,openpeer/libvpx_new,shacklettbp/aom,shacklettbp/aom,VTCSecureLLC/libvpx,mbebenita/aom,jmvalin/aom,jmvalin/aom,luctrudeau/aom,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,luctrudeau/aom,mwgoldsmith/libvpx,shacklettbp/aom,liqianggao/libvpx,VTCSecureLLC/libvpx,GrokImageCompression/aom,shacklettbp/aom,mwgoldsmith/vpx,mwgoldsmith/libvpx,VTCSecureLLC/libvpx,mbebenita/aom,ittiamvpx/libvpx-1,shacklettbp/aom,thdav/aom,mbebenita/aom,felipebetancur/libvpx,VTCSecureLLC/libvpx,mwgoldsmith/libvpx,mbebenita/aom,felipebetancur/libvpx,smarter/aom,zofuthan/libvpx,thdav/aom,smarter/aom,ittiamvpx/libvpx-1,felipebetancur/libvpx,VTCSecureLLC/libvpx,zofuthan/libvpx,luctrudeau/aom,openpeer/libvpx_new,ittiamvpx/libvpx-1,jmvalin/aom,luctrudeau/aom,mbebenita/aom,luctrudeau/aom,smarter/aom,ittiamvpx/libvpx-1,ShiftMediaProject/libvpx,GrokImageCompression/aom,Topopiccione/libvpx,mwgoldsmith/vpx,webmproject/libvpx,webmproject/libvpx,mbebenita/aom,webmproject/libvpx,liqianggao/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,mwgoldsmith/libvpx,shacklettbp/aom,openpeer/libvpx_new,felipebetancur/libvpx,GrokImageCompression/aom,mwgoldsmith/vpx,webmproject/libvpx,Topopiccione/libvpx,Topopiccione/libvpx