On gpu, use max(r,g,b) for coverage alpha in LCD and update lcd blend gm.

src/gpu/effects/GrDistanceFieldGeoProc.cpp

Index: src/gpu/effects/GrDistanceFieldGeoProc.cpp
diff --git a/src/gpu/effects/GrDistanceFieldGeoProc.cpp b/src/gpu/effects/GrDistanceFieldGeoProc.cpp
index 08fa5ac6d55eebfd0b288716e23f991c367121bb..4f8da160e7a11e907180b0e6b592c9e4e5c3d1e0 100644
--- a/src/gpu/effects/GrDistanceFieldGeoProc.cpp
+++ b/src/gpu/effects/GrDistanceFieldGeoProc.cpp
@@ -633,8 +633,10 @@ public:
 
         fsBuilder->codeAppend(
                       "vec4 val = vec4(smoothstep(vec3(-afwidth), vec3(afwidth), distance), 1.0);");
+        // set alpha to be max of rgb coverage
+        fsBuilder->codeAppend("val.a = max(max(val.r, val.g), val.b);");
 
-        fsBuilder->codeAppendf("%s = vec4(val);", args.fOutputCoverage);
+        fsBuilder->codeAppendf("%s = val;", args.fOutputCoverage);
     }
 
     void setData(const GrGLProgramDataManager& pdman,