Use threshold of 1 texture coord value per pixel w/ nearest neighbor.

src/effects/SkAlphaThresholdFilter.cpp

Index: src/effects/SkAlphaThresholdFilter.cpp
diff --git a/src/effects/SkAlphaThresholdFilter.cpp b/src/effects/SkAlphaThresholdFilter.cpp
index 04504b5f8ba3715c9b7b906998eda0f4ebe2ff61..2703a245e058f9bfc63b2b5fa67f21aac4b5d06d 100644
--- a/src/effects/SkAlphaThresholdFilter.cpp
+++ b/src/effects/SkAlphaThresholdFilter.cpp
@@ -87,10 +87,12 @@ private:
         : fInnerThreshold(innerThreshold)
         , fOuterThreshold(outerThreshold)
         , fImageCoordTransform(kLocal_GrCoordSet,
-                               GrCoordTransform::MakeDivByTextureWHMatrix(texture), texture)
+                               GrCoordTransform::MakeDivByTextureWHMatrix(texture), texture,
+                               GrTextureParams::kNone_FilterMode)
         , fImageTextureAccess(texture)
         , fMaskCoordTransform(kLocal_GrCoordSet,
-                              GrCoordTransform::MakeDivByTextureWHMatrix(maskTexture), maskTexture)
+                              GrCoordTransform::MakeDivByTextureWHMatrix(maskTexture), maskTexture,
+                              GrTextureParams::kNone_FilterMode)
         , fMaskTextureAccess(maskTexture) {
         this->initClassID<AlphaThresholdEffect>();
         this->addCoordTransform(&fImageCoordTransform);