Optimize backwardreferences
Optimize backwardreferences

Optimize backwardreferences (about 0.1% byte savings) with almost same
compression speed (3% faster on defaut compression settings).
1.) Simplified iteration logic for HashChainFindCopy.
    - Remapped the iter_max constant.
2.) Simplified main for loop for BackwardReferencesHashChain
    - Removed 'if' conditions for corner cases in the main loop.
    - Refactored the method(AddSingleLiteral) for adding one pixel.

Change-Id: I1bc44832fd81f11e714868a13e606c8f83157e64

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