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 6f9fefce1ba112af36e5c920aa3f343f2442f094..13a745efe0760b6836df279728f5a016d24ba69e 100644
--- a/src/effects/SkAlphaThresholdFilter.cpp
+++ b/src/effects/SkAlphaThresholdFilter.cpp
@@ -48,6 +48,7 @@ SkImageFilter* SkAlphaThresholdFilter::Create(const SkRegion& region,
 #include "GrFragmentProcessor.h"
 #include "GrInvariantOutput.h"
 #include "GrTextureAccess.h"
+#include "effects/GrPorterDuffXferProcessor.h"
 
 #include "SkGr.h"
 
@@ -281,7 +282,7 @@ bool SkAlphaThresholdFilterImpl::asFragmentProcessor(GrFragmentProcessor** fp,
         {
             GrContext::AutoRenderTarget art(context, maskTexture->asRenderTarget());
             GrPaint grPaint;
-            grPaint.setBlendFunc(kOne_GrBlendCoeff, kZero_GrBlendCoeff);
+            grPaint.setPorterDuffXPFactory(SkXfermode::kSrc_Mode);
             SkRegion::Iterator iter(fRegion);
             context->clear(NULL, 0x0, true, maskTexture->asRenderTarget());