Avoid re-rendering stencil clip for every draw with reducable clip stack

src/gpu/SkGpuDevice.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkGpuDevice.h"
 
 #include "effects/GrTextureDomainEffect.h"
@@ skipping 326 matching lines @@
         config = config8888_to_grconfig_and_flags(config8888, &flags);
     } else {
         flags = 0;
         config= SkBitmapConfig2GrPixelConfig(bitmap.config());
     }
 
     fRenderTarget->writePixels(x, y, bitmap.width(), bitmap.height(),
                                config, bitmap.getPixels(), bitmap.rowBytes(), flags);
 }
 
-namespace {
-void purgeClipCB(int genID, void* ) {
-
-    if (SkClipStack::kInvalidGenID == genID ||
-        SkClipStack::kEmptyGenID == genID ||
-        SkClipStack::kWideOpenGenID == genID) {
-        // none of these cases will have a cached clip mask
-        return;
-    }
-
-}
-};
-
 void SkGpuDevice::onAttachToCanvas(SkCanvas* canvas) {
     INHERITED::onAttachToCanvas(canvas);
 
     // Canvas promises that this ptr is valid until onDetachFromCanvas is called
     fClipData.fClipStack = canvas->getClipStack();
-
-    fClipData.fClipStack->addPurgeClipCallback(purgeClipCB, fContext);
 }
 
 void SkGpuDevice::onDetachFromCanvas() {
     INHERITED::onDetachFromCanvas();
-
-    // TODO: iterate through the clip stack and clean up any cached clip masks
-    fClipData.fClipStack->removePurgeClipCallback(purgeClipCB, fContext);
-
     fClipData.fClipStack = NULL;
 }
 
 // call this every draw call, to ensure that the context reflects our state,
 // and not the state from some other canvas/device
 void SkGpuDevice::prepareDraw(const SkDraw& draw, bool forceIdentity) {
     SkASSERT(NULL != fClipData.fClipStack);
 
     fContext->setRenderTarget(fRenderTarget);
 
@@ skipping 1466 matching lines @@
                          GrTexture* texture,
                          bool needClear)
     : SkBitmapDevice(make_bitmap(context, texture->asRenderTarget())) {
 
     SkASSERT(texture && texture->asRenderTarget());
     // This constructor is called from onCreateCompatibleDevice. It has locked the RT in the texture
     // cache. We pass true for the third argument so that it will get unlocked.
     this->initFromRenderTarget(context, texture->asRenderTarget(), true);
     fNeedClear = needClear;
 }