Cleanup and commenting of the sub-pixel interpolation functions. Mostly as a preparation for the upcoming vectorized implementation.
Cleanup and commenting of the sub-pixel interpolation functions. Mostly
as a preparation for the upcoming vectorized implementation.

Change-Id: I0d1629943cee68fcb0ea8b6dd8aef265d050cc7a

WebRTC-Labs/libvpx,kim42083/webm.libvpx,luctrudeau/aom,goodleixiao/vpx,cinema6/libvpx,Acidburn0zzz/webm.libvpx,zofuthan/libvpx,iniwf/webm.libvpx,VTCSecureLLC/libvpx,n4t/libvpx,shareefalis/libvpx,charup/https---github.com-webmproject-libvpx-,shacklettbp/aom,ShiftMediaProject/libvpx,vasilvv/esvp8,jdm/libvpx,liqianggao/libvpx,mwgoldsmith/libvpx,matanbs/vp982,zofuthan/libvpx,thdav/aom,Laknot/libvpx,mbebenita/aom,GrokImageCompression/aom,Suvarna1488/webm.libvpx,mbebenita/aom,cinema6/libvpx,goodleixiao/vpx,kleopatra999/webm.libvpx,ittiamvpx/libvpx,Suvarna1488/webm.libvpx,kalli123/webm.libvpx,kleopatra999/webm.libvpx,GrokImageCompression/aom,kalli123/webm.libvpx,cinema6/libvpx,matanbs/webm.libvpx,gshORTON/webm.libvpx,jdm/libvpx,matanbs/webm.libvpx,liqianggao/libvpx,mbebenita/aom,abwiz0086/webm.libvpx,ittiamvpx/libvpx-1,Laknot/libvpx,VTCSecureLLC/libvpx,WebRTC-Labs/libvpx,mwgoldsmith/vpx,matanbs/vp982,GrokImageCompression/aom,VTCSecureLLC/libvpx,Distrotech/libvpx,pcwalton/libvpx,luctrudeau/aom,reimaginemedia/webm.libvpx,liqianggao/libvpx,luctrudeau/aom,Distrotech/libvpx,running770/libvpx,smarter/aom,iniwf/webm.libvpx,goodleixiao/vpx,ittiamvpx/libvpx-1,lyx2014/libvpx_c,Topopiccione/libvpx,GrokImageCompression/aom,Distrotech/libvpx,kleopatra999/webm.libvpx,pcwalton/libvpx,shyamalschandra/libvpx,iniwf/webm.libvpx,smarter/aom,shyamalschandra/libvpx,mwgoldsmith/vpx,mwgoldsmith/vpx,turbulenz/libvpx,abwiz0086/webm.libvpx,mbebenita/aom,cinema6/libvpx,running770/libvpx,luctrudeau/aom,Suvarna1488/webm.libvpx,hsueceumd/test_hui,abwiz0086/webm.libvpx,shareefalis/libvpx,ittiamvpx/libvpx,shareefalis/libvpx,thdav/aom,vasilvv/esvp8,matanbs/vp982,jacklicn/webm.libvpx,zofuthan/libvpx,mwgoldsmith/vpx,iniwf/webm.libvpx,shareefalis/libvpx,turbulenz/libvpx,turbulenz/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,felipebetancur/libvpx,mwgoldsmith/libvpx,jmvalin/aom,pcwalton/libvpx,smarter/aom,liqianggao/libvpx,matanbs/webm.libvpx,zofuthan/libvpx,kalli123/webm.libvpx,thdav/aom,turbulenz/libvpx,charup/https---github.com-webmproject-libvpx-,jmvalin/aom,altogother/webm.libvpx,charup/https---github.com-webmproject-libvpx-,stewnorriss/libvpx,mwgoldsmith/vpx,Laknot/libvpx,reimaginemedia/webm.libvpx,webmproject/libvpx,matanbs/webm.libvpx,ShiftMediaProject/libvpx,WebRTC-Labs/libvpx,Maria1099/webm.libvpx,liqianggao/libvpx,kim42083/webm.libvpx,vasilvv/esvp8,vasilvv/esvp8,kleopatra999/webm.libvpx,gshORTON/webm.libvpx,running770/libvpx,Acidburn0zzz/webm.libvpx,vasilvv/esvp8,goodleixiao/vpx,sanyaade-teachings/libvpx,thdav/aom,pcwalton/libvpx,altogother/webm.libvpx,lyx2014/libvpx_c,goodleixiao/vpx,shyamalschandra/libvpx,kim42083/webm.libvpx,n4t/libvpx,liqianggao/libvpx,mbebenita/aom,Acidburn0zzz/webm.libvpx,felipebetancur/libvpx,jdm/libvpx,turbulenz/libvpx,Suvarna1488/webm.libvpx,openpeer/libvpx_new,felipebetancur/libvpx,kalli123/webm.libvpx,cinema6/libvpx,running770/libvpx,jacklicn/webm.libvpx,reimaginemedia/webm.libvpx,running770/libvpx,hsueceumd/test_hui,Maria1099/webm.libvpx,jacklicn/webm.libvpx,jacklicn/webm.libvpx,Laknot/libvpx,jmvalin/aom,ittiamvpx/libvpx-1,Maria1099/webm.libvpx,ittiamvpx/libvpx,Laknot/libvpx,n4t/libvpx,sanyaade-teachings/libvpx,reimaginemedia/webm.libvpx,jacklicn/webm.libvpx,thdav/aom,jdm/libvpx,webmproject/libvpx,Maria1099/webm.libvpx,ittiamvpx/libvpx,charup/https---github.com-webmproject-libvpx-,mbebenita/aom,lyx2014/libvpx_c,Distrotech/libvpx,VTCSecureLLC/libvpx,kim42083/webm.libvpx,vasilvv/esvp8,lyx2014/libvpx_c,shacklettbp/aom,stewnorriss/libvpx,Maria1099/webm.libvpx,iniwf/webm.libvpx,ittiamvpx/libvpx,charup/https---github.com-webmproject-libvpx-,matanbs/webm.libvpx,jmvalin/aom,mwgoldsmith/libvpx,felipebetancur/libvpx,stewnorriss/libvpx,ittiamvpx/libvpx-1,sanyaade-teachings/libvpx,matanbs/vp982,stewnorriss/libvpx,lyx2014/libvpx_c,luctrudeau/aom,Maria1099/webm.libvpx,Topopiccione/libvpx,reimaginemedia/webm.libvpx,pcwalton/libvpx,altogother/webm.libvpx,mbebenita/aom,VTCSecureLLC/libvpx,kalli123/webm.libvpx,hsueceumd/test_hui,altogother/webm.libvpx,iniwf/webm.libvpx,jdm/libvpx,vasilvv/esvp8,shyamalschandra/libvpx,gshORTON/webm.libvpx,turbulenz/libvpx,openpeer/libvpx_new,n4t/libvpx,stewnorriss/libvpx,Acidburn0zzz/webm.libvpx,goodleixiao/vpx,kleopatra999/webm.libvpx,shacklettbp/aom,shacklettbp/aom,Distrotech/libvpx,sanyaade-teachings/libvpx,webmproject/libvpx,Acidburn0zzz/webm.libvpx,ittiamvpx/libvpx-1,luctrudeau/aom,altogother/webm.libvpx,kim42083/webm.libvpx,openpeer/libvpx_new,shacklettbp/aom,felipebetancur/libvpx,webmproject/libvpx,Acidburn0zzz/webm.libvpx,pcwalton/libvpx,smarter/aom,jmvalin/aom,hsueceumd/test_hui,Laknot/libvpx,Topopiccione/libvpx,n4t/libvpx,matanbs/webm.libvpx,Topopiccione/libvpx,lyx2014/libvpx_c,shareefalis/libvpx,Suvarna1488/webm.libvpx,thdav/aom,gshORTON/webm.libvpx,charup/https---github.com-webmproject-libvpx-,shacklettbp/aom,turbulenz/libvpx,WebRTC-Labs/libvpx,Distrotech/libvpx,ShiftMediaProject/libvpx,jacklicn/webm.libvpx,felipebetancur/libvpx,webmproject/libvpx,jmvalin/aom,mbebenita/aom,kim42083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