Dynamically create stencil buffer when needed.

src/effects/SkAlphaThresholdFilter.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkAlphaThresholdFilter.h"
 #include "SkBitmap.h"
 #include "SkReadBuffer.h"
@@ skipping 255 matching lines @@
                                                      const SkMatrix& in_matrix,
                                                      const SkIRect&) const {
     if (fp) {
         GrContext* context = texture->getContext();
         GrSurfaceDesc maskDesc;
         if (context->isConfigRenderable(kAlpha_8_GrPixelConfig, false)) {
             maskDesc.fConfig = kAlpha_8_GrPixelConfig;
         } else {
             maskDesc.fConfig = kRGBA_8888_GrPixelConfig;
         }
-        maskDesc.fFlags = kRenderTarget_GrSurfaceFlag | kNoStencil_GrSurfaceFlag;
+        maskDesc.fFlags = kRenderTarget_GrSurfaceFlag;
         // Add one pixel of border to ensure that clamp mode will be all zeros
         // the outside.
         maskDesc.fWidth = texture->width();
         maskDesc.fHeight = texture->height();
         SkAutoTUnref<GrTexture> maskTexture(
             context->refScratchTexture(maskDesc, GrContext::kApprox_ScratchTexMatch));
         if (!maskTexture) {
             return false;
         }
 
@@ skipping 89 matching lines @@
 }
 
 #ifndef SK_IGNORE_TO_STRING
 void SkAlphaThresholdFilterImpl::toString(SkString* str) const {
     str->appendf("SkAlphaThresholdImageFilter: (");
     str->appendf("inner: %f outer: %f", fInnerThreshold, fOuterThreshold);
     str->append(")");
 }
 #endif