AnimEncoder lib cleanup: prev to prev canvas not needed.
AnimEncoder lib cleanup: prev to prev canvas not needed.

Given that we decided to only handle frame disposal for output WebP
internally,
only current and previous canvas need to be maintained.

Change-Id: I625293bed5aeb5aabf4eca779f6ec3ee84c9ff2a

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