alpha threshold bitmap shader

src/gpu/SkGpuDevice.cpp

Index: src/gpu/SkGpuDevice.cpp
diff --git a/src/gpu/SkGpuDevice.cpp b/src/gpu/SkGpuDevice.cpp
index 571b8a0a7d91206159ec71fcddbafb65c1581cbd..ea8a06e632683e7f0ebb75dad23d3b48efa24425 100644
--- a/src/gpu/SkGpuDevice.cpp
+++ b/src/gpu/SkGpuDevice.cpp
@@ -535,6 +535,11 @@ inline bool skPaint2GrPaintShader(SkGpuDevice* dev,
         return skPaint2GrPaintNoShader(dev, skPaint, false, constantColor, grPaint);
     }
 
+    // SkShader::asNewEffect() may do offscreen rendering. Setup default drawing state
+    // Also require shader to set the render target .
+    GrContext::AutoWideOpenIdentityDraw awo(dev->context(), NULL);
+    GrContext::AutoRenderTarget(dev->context(), NULL);
+
     // setup the shader as the first color effect on the paint
     SkAutoTUnref<GrEffectRef> effect(shader->asNewEffect(dev->context(), skPaint));
     if (NULL != effect.get()) {