discard when coverage is <= 0 in XP dst copy.
discard when coverage is <= 0 in XP dst copy.

Darken was 5-20% slower with this CL, though it might improve things a bit to only look at one component of the vector.

BUG=skia:

Review URL: https://codereview.chromium.org/915633002

HalCanary/skia-hc,Igalia/skia,geekboxzone/mmallow_external_skia,Infinitive-OS/platform_external_skia,Igalia/skia,ominux/skia,UBERMALLOW/external_skia,MarshedOut/android_external_skia,TeamExodus/external_skia,Jichao/skia,scroggo/skia,Infinitive-OS/platform_external_skia,samuelig/skia,rubenvb/skia,AOSP-YU/platform_external_skia,invisiblek/android_external_skia,nvoron23/skia,MonkeyZZZZ/platform_external_skia,UBERMALLOW/external_skia,AOSPB/external_skia,AOSP-YU/platform_external_skia,TeamTwisted/external_skia,rubenvb/skia,AOSP-YU/platform_external_skia,geekboxzone/mmallow_external_skia,geekboxzone/mmallow_external_skia,Igalia/skia,Infinitive-OS/platform_external_skia,PAC-ROM/android_external_skia,scroggo/skia,BrokenROM/external_skia,Hikari-no-Tenshi/android_external_skia,google/skia,boulzordev/android_external_skia,TeamExodus/external_skia,PAC-ROM/android_external_skia,samuelig/skia,BrokenROM/external_skia,shahrzadmn/skia,timduru/platform-external-skia,MarshedOut/android_external_skia,AOSPB/external_skia,PAC-ROM/android_external_skia,pcwalton/skia,scroggo/skia,aosp-mirror/platform_external_skia,nvoron23/skia,qrealka/skia-hc,AOSPB/external_skia,timduru/platform-external-skia,geekboxzone/mmallow_external_skia,Hikari-no-Tenshi/android_external_skia,MonkeyZZZZ/platform_external_skia,rubenvb/skia,nvoron23/skia,VRToxin-AOSP/android_external_skia,nfxosp/platform_external_skia,Jichao/skia,amyvmiwei/skia,YUPlayGodDev/platform_external_skia,VRToxin-AOSP/android_external_skia,TeamTwisted/external_skia,nfxosp/platform_external_skia,AOSP-YU/platform_external_skia,spezi77/android_external_skia,AOSP-YU/platform_external_skia,TeamExodus/external_skia,OneRom/external_skia,spezi77/android_external_skia,spezi77/android_external_skia,shahrzadmn/skia,boulzordev/android_external_skia,MinimalOS-AOSP/platform_external_skia,Jichao/skia,MonkeyZZZZ/platform_external_skia,scroggo/skia,UBERMALLOW/external_skia,rubenvb/skia,noselhq/skia,noselhq/skia,samuelig/skia,PAC-ROM/android_external_skia,amyvmiwei/skia,Hikari-no-Tenshi/android_external_skia,MinimalOS-AOSP/platform_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,AOSP-YU/platform_external_skia,invisiblek/android_external_skia,aosp-mirror/platform_external_skia,timduru/platform-external-skia,invisiblek/android_external_skia,amyvmiwei/skia,MarshedOut/android_external_skia,HalCanary/skia-hc,google/skia,pcwalton/skia,rubenvb/skia,qrealka/skia-hc,OneRom/external_skia,MonkeyZZZZ/platform_external_skia,vanish87/skia,vanish87/skia,shahrzadmn/skia,scroggo/skia,ominux/skia,vanish87/skia,HalCanary/skia-hc,qrealka/skia-hc,TeamExodus/external_skia,qrealka/skia-hc,aosp-mirror/platform_external_skia,AOSPB/external_skia,todotodoo/skia,pcwalton/skia,samuelig/skia,geekboxzone/mmallow_external_skia,invisiblek/android_external_skia,vanish87/skia,YUPlayGodDev/platform_external_skia,ominux/skia,nfxosp/platform_external_skia,pcwalton/skia,boulzordev/android_external_skia,nvoron23/skia,spezi77/android_external_skia,nvoron23/skia,MarshedOut/android_external_skia,HalCanary/skia-hc,timduru/platform-external-skia,nfxosp/platform_external_skia,qrealka/skia-hc,Igalia/skia,shahrzadmn/skia,nfxosp/platform_external_skia,VRToxin-AOSP/android_external_skia,TeamTwisted/external_skia,todotodoo/skia,pcwalton/skia,Igalia/skia,samuelig/skia,w3nd1go/android_external_skia,todotodoo/skia,aosp-mirror/platform_external_skia,tmpvar/skia.cc,boulzordev/android_external_skia,MonkeyZZZZ/platform_external_skia,tmpvar/skia.cc,ominux/skia,MinimalOS-AOSP/platform_external_skia,noselhq/skia,HalCanary/skia-hc,OneRom/external_skia,google/skia,AOSPB/external_skia,shahrzadmn/skia,VRToxin-AOSP/android_external_skia,google/skia,UBERMALLOW/external_skia,rubenvb/skia,rubenvb/skia,AOSPB/external_skia,w3nd1go/android_external_skia,UBERMALLOW/external_skia,pcwalton/skia,YUPlayGodDev/platform_external_skia,todotodoo/skia,OneRom/external_skia,geekboxzone/mmallow_external_skia,nfxosp/platform_external_skia,invisiblek/android_external_skia,VRToxin-AOSP/android_external_skia,MarshedOut/android_external_skia,OneRom/external_skia,HalCanary/skia-hc,VRToxin-AOSP/android_external_skia,tmpvar/skia.cc,w3nd1go/android_external_skia,geekboxzone/mmallow_external_skia,amyvmiwei/skia,geekboxzone/mmallow_external_skia,scroggo/skia,TeamExodus/external_skia,VRToxin-AOSP/android_external_skia,Jichao/skia,boulzordev/android_external_skia,aosp-mirror/platform_external_skia,OneRom/external_skia,aosp-mirror/platform_external_skia,TeamExodus/external_skia,amyvmiwei/skia,TeamTwisted/external_skia,qrealka/skia-hc,scroggo/skia,AOSP-YU/platform_external_skia,rubenvb/skia,Jichao/skia,Jichao/skia,qrealka/skia-hc,Hikari-no-Tenshi/android_external_skia,noselhq/skia,MinimalOS-AOSP/platform_external_skia,MarshedOut/android_external_skia,Hikari-no-Tenshi/android_external_skia,TeamExodus/external_skia,TeamTwisted/external_skia,w3nd1go/android_external_skia,nvoron23/skia,google/skia,amyvmiwei/skia,scroggo/skia,qrealka/skia-hc,TeamTwisted/external_skia,YUPlayGodDev/platform_external_skia,nvoron23/skia,tmpvar/skia.cc,samuelig/skia,ominux/skia,ominux/skia,boulzordev/android_external_skia,todotodoo/skia,PAC-ROM/android_external_skia,pcwalton/skia,Igalia/skia,tmpvar/skia.cc,BrokenROM/external_skia,google/skia,vanish87/skia,todotodoo/skia,PAC-ROM/android_external_skia,nfxosp/platform_external_skia,TeamTwisted/external_skia,AOSP-YU/platform_external_skia,invisiblek/android_external_skia,Jichao/skia,MarshedOut/android_external_skia,timduru/platform-external-skia,nfxosp/platform_external_skia,MonkeyZZZZ/platform_external_skia,Infinitive-OS/platform_external_skia,aosp-mirror/platform_external_skia,Infinitive-OS/platform_external_skia,VRToxin-AOSP/android_external_skia,aosp-mirror/platform_external_skia,MinimalOS-AOSP/platform_external_skia,aosp-mirror/platform_external_skia,OneRom/external_skia,Jichao/skia,TeamExodus/external_skia,pcwalton/skia,VRToxin-AOSP/android_external_skia,YUPlayGodDev/platform_external_skia,boulzordev/android_external_skia,timduru/platform-external-skia,Igalia/skia,HalCanary/skia-hc,w3nd1go/android_external_skia,shahrzadmn/skia,TeamTwisted/external_skia,AOSPB/external_skia,YUPlayGodDev/platform_external_skia,AOSPB/external_skia,amyvmiwei/skia,geekboxzone/mmallow_external_skia,noselhq/skia,UBERMALLOW/external_skia,w3nd1go/android_external_skia,tmpvar/skia.cc,YUPlayGodDev/platform_external_skia,YUPlayGodDev/platform_external_skia,timduru/platform-external-skia,UBERMALLOW/external_skia,samuelig/skia,vanish87/skia,Jichao/skia,shahrzadmn/skia,boulzordev/android_external_skia,ominux/skia,noselhq/skia,MinimalOS-AOSP/platform_external_skia,invisiblek/android_external_skia,Infinitive-OS/platform_external_skia,MarshedOut/android_external_skia,BrokenROM/external_skia,OneRom/external_skia,vanish87/skia,spezi77/android_external_skia,HalCanary/skia-hc,tmpvar/skia.cc,YUPlayGodDev/platform_external_skia,w3nd1go/android_external_skia,Hikari-no-Tenshi/android_external_skia,UBERMALLOW/external_skia,Hikari-no-Tenshi/android_external_skia,nvoron23/skia,MinimalOS-AOSP/platform_external_skia,google/skia,tmpvar/skia.cc,Infinitive-OS/platform_external_skia,todotodoo/skia,TeamExodus/external_skia,ominux/skia,AOSPB/external_skia,MonkeyZZZZ/platform_external_skia,w3nd1go/android_external_skia,tmpvar/skia.cc,Infinitive-OS/platform_external_skia,OneRom/external_skia,PAC-ROM/android_external_skia,samuelig/skia,AOSP-YU/platform_external_skia,google/skia,BrokenROM/external_skia,pcwalton/skia,MinimalOS-AOSP/platform_external_skia,todotodoo/skia,boulzordev/android_external_skia,Infinitive-OS/platform_external_skia,vanish87/skia,todotodoo/skia,BrokenROM/external_skia,invisiblek/android_external_skia,MinimalOS-AOSP/platform_external_skia,PAC-ROM/android_external_skia,nfxosp/platform_external_skia,noselhq/skia,Hikari-no-Tenshi/android_external_skia,Igalia/skia,MonkeyZZZZ/platform_external_skia,HalCanary/skia-hc,google/skia,UBERMALLOW/external_skia,MarshedOut/android_external_skia,vanish87/skia,amyvmiwei/skia,noselhq/skia,google/skia,BrokenROM/external_skia,rubenvb/skia,shahrzadmn/skia,nvoron23/skia,shahrzadmn/skia,spezi77/android_external_skia,TeamTwisted/external_skia,BrokenROM/external_skia,PAC-ROM/android_external_skia,ominux/skia,noselhq/skia,HalCanary/skia-hc,MonkeyZZZZ/platform_external_skia,w3nd1go/android_external_skia