vwebp/animation: fix background dispose
vwebp/animation: fix background dispose

buffer the last frame's details to perform DISPOSE_BACKGROUND on the
image's area, rather than the entire canvas.

also fixes transparent backgrounds with animated images

Change-Id: I53e4d70c441e1eeb136f1d01e7c88de4f9ecff53

appunite/libwebp,PKRoma/libwebp,altogother/webm.libwebp,matanbs/webm.libwebp,redengineer/libwebp,ya7lelkom/libwebp,redengineer/libwebp,PKRoma/libwebp,keane83/BLE_IM,Aexyn/libwebp,Aexyn/libwebp,kleopatra999/webm.libwebp,Suvarna1488/webm.libwebp,imazen/libwebp,jiehu5114/libwebp,cysp/libwebp,kleopatra999/webm.libwebp,kim42083/webm.libwebp,reimaginemedia/webm.libwebp,kleopatra999/webm.libwebp,ya7lelkom/libwebp,kleopatra999/webm.libwebp,jacklicn/libwebp,appunite/libwebp,yohunl/libwebp,webmproject/libwebp,rxl194/libwebp,yohunl/libwebp,Acidburn0zzz/webm.libwebp,Maria1099/webm.libwebp,ericmckean/webm.libwebp,keane83/BLE_IM,kim42083/webm.libwebp,matanbs/webm.libwebp,redengineer/libwebp,Suvarna1488/webm.libwebp,ttyangf/libwebp,ericmckean/webm.libwebp,webmproject/libwebp,Acidburn0zzz/webm.libwebp,Tekserve/libwebp,zofuthan/libwebp,zofuthan/libwebp,zofuthan/libwebp,Acidburn0zzz/webm.libwebp,PKRoma/libwebp,jiehu5114/libwebp,webmproject/libwebp,Maria1099/webm.libwebp,zofuthan/libwebp,Acidburn0zzz/webm.libwebp,ttyangf/libwebp,comoyo/gonzo-libwebp,gshORTON/webm.libwebp,Suvarna1488/webm.libwebp,reimaginemedia/webm.libwebp,reimaginemedia/webm.libwebp,rxl194/libwebp,jiehu5114/libwebp,ya7lelkom/libwebp,kalli123/webm.libwebp,redengineer/libwebp,jacklicn/libwebp,jiehu5114/libwebp,ericmckean/webm.libwebp,reimaginemedia/webm.libwebp,rxl194/libwebp,iniwf/webm.libwebp,altogother/webm.libwebp,kleopatra999/webm.libwebp,iniwf/webm.libwebp,Tekserve/libwebp,altogother/webm.libwebp,webmproject/libwebp,kim42083/webm.libwebp,appunite/libwebp,iniwf/webm.libwebp,Maria1099/webm.libwebp,jiehu5114/libwebp,ttyangf/libwebp,keane83/BLE_IM,yohunl/libwebp,Aexyn/libwebp,abwiz0086/webm.libwebp,kalli123/webm.libwebp,kim42083/webm.libwebp,imazen/libwebp,Aexyn/libwebp,jacklicn/libwebp,cysp/libwebp,Aexyn/libwebp,Acidburn0zzz/webm.libwebp,ya7lelkom/libwebp,keane83/BLE_IM,abwiz0086/webm.libwebp,kalli123/webm.libwebp,ericmckean/webm.libwebp,yohunl/libwebp,gshORTON/webm.libwebp,Suvarna1488/webm.libwebp,gshORTON/webm.libwebp,matanbs/webm.libwebp,comoyo/gonzo-libwebp,matanbs/webm.libwebp,comoyo/gonzo-libwebp,Distrotech/libwebp,abwiz0086/webm.libwebp,PKRoma/libwebp,reimaginemedia/webm.libwebp,webmproject/libwebp,comoyo/gonzo-libwebp,ttyangf/libwebp,kalli123/webm.libwebp,Distrotech/libwebp,rxl194/libwebp,ttyangf/libwebp,comoyo/gonzo-libwebp,kim42083/webm.libwebp,ericmckean/webm.libwebp,Maria1099/webm.libwebp,gshORTON/webm.libwebp,iniwf/webm.libwebp,PKRoma/libwebp,Maria1099/webm.libwebp,gshORTON/webm.libwebp,comoyo/gonzo-libwebp,matanbs/webm.libwebp,appunite/libwebp,Suvarna1488/webm.libwebp,abwiz0086/webm.libwebp,imazen/libwebp,Tekserve/libwebp,imazen/libwebp,jacklicn/libwebp,Tekserve/libwebp,appunite/libwebp,jacklicn/libwebp,iniwf/webm.libwebp,rxl194/libwebp,imazen/libwebp,keane83/BLE_IM,kalli123/webm.libwebp,abwiz0086/webm.libwebp,comoyo/gonzo-libwebp,altogother/webm.libwebp,cysp/libwebp,redengineer/libwebp,altogother/webm.libwebp,Distrotech/libwebp,PKRoma/libwebp,ya7lelkom/libwebp,zofuthan/libwebp,appunite/libwebp,yohunl/libwebp,cysp/libwebp