modulate alpha-compression effort according to config.method
modulate alpha-compression effort according to config.method

we vary linearly lossless-method between 0 and 6,
and lossless-quality between 50 and 100, so that encoding
speed can go from 'quite fast' to 'rather slow'.
Impact on size is moderate, but visible.

Change-Id: I0b7917e7170eb50258afb1a4e248028cd9e9207d

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