Premultiply with alpha during U/V downsampling
Premultiply with alpha during U/V downsampling

This prevents the 'alpha-leak' reported in issue #220

Speed-diff is kept minimal.

Change-Id: I1976de5e6de7cfcec89a54df9233c1a6586a5846

PKRoma/libwebp,PKRoma/libwebp,matanbs/webm.libwebp,gshORTON/webm.libwebp,imazen/libwebp,kalli123/webm.libwebp,rxl194/libwebp,comoyo/gonzo-libwebp,kim42083/webm.libwebp,matanbs/webm.libwebp,kleopatra999/webm.libwebp,yohunl/libwebp,gshORTON/webm.libwebp,matanbs/webm.libwebp,abwiz0086/webm.libwebp,yohunl/libwebp,webmproject/libwebp,abwiz0086/webm.libwebp,iniwf/webm.libwebp,Maria1099/webm.libwebp,kim42083/webm.libwebp,rxl194/libwebp,Acidburn0zzz/webm.libwebp,comoyo/gonzo-libwebp,redengineer/libwebp,jacklicn/libwebp,abwiz0086/webm.libwebp,ttyangf/libwebp,ttyangf/libwebp,ya7lelkom/libwebp,jiehu5114/libwebp,jacklicn/libwebp,imazen/libwebp,imazen/libwebp,kleopatra999/webm.libwebp,ericmckean/webm.libwebp,Aexyn/libwebp,kalli123/webm.libwebp,reimaginemedia/webm.libwebp,abwiz0086/webm.libwebp,jiehu5114/libwebp,webmproject/libwebp,Aexyn/libwebp,yohunl/libwebp,altogother/webm.libwebp,Suvarna1488/webm.libwebp,gshORTON/webm.libwebp,kalli123/webm.libwebp,kalli123/webm.libwebp,reimaginemedia/webm.libwebp,zofuthan/libwebp,yohunl/libwebp,comoyo/gonzo-libwebp,Acidburn0zzz/webm.libwebp,rxl194/libwebp,Suvarna1488/webm.libwebp,iniwf/webm.libwebp,Suvarna1488/webm.libwebp,PKRoma/libwebp,PKRoma/libwebp,Maria1099/webm.libwebp,webmproject/libwebp,jiehu5114/libwebp,imazen/libwebp,rxl194/libwebp,zofuthan/libwebp,redengineer/libwebp,kim42083/webm.libwebp,kleopatra999/webm.libwebp,kim42083/webm.libwebp,ttyangf/libwebp,reimaginemedia/webm.libwebp,ericmckean/webm.libwebp,ttyangf/libwebp,Acidburn0zzz/webm.libwebp,abwiz0086/webm.libwebp,Aexyn/libwebp,ya7lelkom/libwebp,webmproject/libwebp,iniwf/webm.libwebp,redengineer/libwebp,comoyo/gonzo-libwebp,Aexyn/libwebp,kleopatra999/webm.libwebp,Acidburn0zzz/webm.libwebp,ya7lelkom/libwebp,kalli123/webm.libwebp,redengineer/libwebp,comoyo/gonzo-libwebp,kleopatra999/webm.libwebp,zofuthan/libwebp,Suvarna1488/webm.libwebp,ericmckean/webm.libwebp,jiehu5114/libwebp,altogother/webm.libwebp,ya7lelkom/libwebp,altogother/webm.libwebp,altogother/webm.libwebp,altogother/webm.libwebp,zofuthan/libwebp,kim42083/webm.libwebp,ttyangf/libwebp,jacklicn/libwebp,ericmckean/webm.libwebp,matanbs/webm.libwebp,imazen/libwebp,gshORTON/webm.libwebp,zofuthan/libwebp,comoyo/gonzo-libwebp,ericmckean/webm.libwebp,yohunl/libwebp,PKRoma/libwebp,reimaginemedia/webm.libwebp,Suvarna1488/webm.libwebp,rxl194/libwebp,reimaginemedia/webm.libwebp,Aexyn/libwebp,jacklicn/libwebp,Maria1099/webm.libwebp,Acidburn0zzz/webm.libwebp,Maria1099/webm.libwebp,iniwf/webm.libwebp,matanbs/webm.libwebp,jacklicn/libwebp,comoyo/gonzo-libwebp,jiehu5114/libwebp,gshORTON/webm.libwebp,Maria1099/webm.libwebp,PKRoma/libwebp,webmproject/libwebp,ya7lelkom/libwebp,redengineer/libwebp,iniwf/webm.libwebp