contrast: process channels separately
contrast: process channels separately

Remove the SOX_EFF_MCHAN flag for the contrast effect. Processing all
channels at once isn't necessary for contrast (even though it is more
efficient) and more prone to bugs.

jacksonh/sox,pcqpcq/sox,Distrotech/sox,Motiejus/sox,davel/sox,uklauer/sox,cbagwell/sox,Motiejus/sox,pcqpcq/sox,mhartzel/sox_personal_fork,davel/sox,cbagwell/sox,MageSlayer/sox,cbagwell/sox,CaptainHayashi/sox,MageSlayer/sox,jacksonh/sox,Motiejus/sox,mhartzel/sox_personal_fork,davel/sox,uklauer/sox,Motiejus/sox,jacksonh/sox,jacksonh/sox,Distrotech/sox,uklauer/sox,CaptainHayashi/sox,MageSlayer/sox,mhartzel/sox_personal_fork,pcqpcq/sox,CaptainHayashi/sox,Distrotech/sox,MageSlayer/sox,Distrotech/sox,davel/sox,uklauer/sox,MageSlayer/sox,mhartzel/sox_personal_fork,cbagwell/sox,CaptainHayashi/sox,pcqpcq/sox