Make all blending up to GrOptDrawState be handled by the xp/xp factory.

src/effects/SkAlphaThresholdFilter.cpp

Index: src/effects/SkAlphaThresholdFilter.cpp
diff --git a/src/effects/SkAlphaThresholdFilter.cpp b/src/effects/SkAlphaThresholdFilter.cpp
index ffdba322649a30ebd4cec24f37e57364ca038f4c..ae2a18a3905ad0115133551c3f9a14c76ec955ad 100644
--- a/src/effects/SkAlphaThresholdFilter.cpp
+++ b/src/effects/SkAlphaThresholdFilter.cpp
@@ -51,6 +51,7 @@ SkImageFilter* SkAlphaThresholdFilter::Create(const SkRegion& region,
 #include "gl/builders/GrGLProgramBuilder.h"
 #include "GrTBackendProcessorFactory.h"
 #include "GrTextureAccess.h"
+#include "effects/GrPorterDuffXferProcessor.h"
 
 #include "SkGr.h"
 
@@ -281,7 +282,8 @@ bool SkAlphaThresholdFilterImpl::asFragmentProcessor(GrFragmentProcessor** fp,
         {
             GrContext::AutoRenderTarget art(context, maskTexture->asRenderTarget());
             GrPaint grPaint;
-            grPaint.setBlendFunc(kOne_GrBlendCoeff, kZero_GrBlendCoeff);
+            GrXPFactory* xpFactory = GrPorterDuffXPFactory::Create(SkXfermode::kSrc_Mode);
+            grPaint.setXPFactory(xpFactory)->unref();
             SkRegion::Iterator iter(fRegion);
             context->clear(NULL, 0x0, true, maskTexture->asRenderTarget());