lossless: 0.37 % compression density improvement
lossless: 0.37 % compression density improvement

counting the entropy expectation for five different configurations:
palette
non-predicted
non-predicted with subtract green
predicted
predicted with subtract green

and choose the strategy with the smallest expected entropy

Change-Id: Iaaf209c0d565660a54a4f9b3959067afb9951960

zofuthan/libwebp,webmproject/libwebp,yohunl/libwebp,imazen/libwebp,imazen/libwebp,redengineer/libwebp,PKRoma/libwebp,zofuthan/libwebp,ya7lelkom/libwebp,rxl194/libwebp,rxl194/libwebp,rxl194/libwebp,PKRoma/libwebp,yohunl/libwebp,rxl194/libwebp,yohunl/libwebp,Aexyn/libwebp,redengineer/libwebp,Aexyn/libwebp,ya7lelkom/libwebp,ya7lelkom/libwebp,PKRoma/libwebp,ya7lelkom/libwebp,Aexyn/libwebp,Aexyn/libwebp,rxl194/libwebp,ya7lelkom/libwebp,redengineer/libwebp,ttyangf/libwebp,ttyangf/libwebp,ttyangf/libwebp,redengineer/libwebp,webmproject/libwebp,zofuthan/libwebp,PKRoma/libwebp,zofuthan/libwebp,imazen/libwebp,zofuthan/libwebp,PKRoma/libwebp,yohunl/libwebp,imazen/libwebp,ttyangf/libwebp,yohunl/libwebp,webmproject/libwebp,imazen/libwebp,redengineer/libwebp,PKRoma/libwebp,ttyangf/libwebp,Aexyn/libwebp,webmproject/libwebp,webmproject/libwebp