allow canvas to force conservative clips (for speed)

include/core/SkDevice.h

 /*
  * Copyright 2010 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkDevice_DEFINED
 #define SkDevice_DEFINED
 
@@ skipping 354 matching lines @@
     friend class SkDeferredDevice;    // for newSurface
 
     friend class SkSurface_Raster;
 
     // used to change the backend's pixels (and possibly config/rowbytes)
     // but cannot change the width/height, so there should be no change to
     // any clip information.
     // TODO: move to SkBitmapDevice
     virtual void replaceBitmapBackendForRasterSurface(const SkBitmap&) {}
 
+    virtual bool forceConservativeRasterClip() const { return false; }
+
     // just called by SkCanvas when built as a layer
     void setOrigin(int x, int y) { fOrigin.set(x, y); }
     // just called by SkCanvas for saveLayer
     SkBaseDevice* createCompatibleDeviceForSaveLayer(const SkImageInfo&);
 
     virtual SkBaseDevice* onCreateDevice(const SkImageInfo&, Usage) {
         return NULL;
     }
 
     /** Causes any deferred drawing to the device to be completed.
      */
     virtual void flush() {}
 
     virtual SkImageFilter::Cache* getImageFilterCache() { return NULL; }
 
     SkIPoint    fOrigin;
     SkMetaData* fMetaData;
 
 #ifdef SK_DEBUG
     bool        fAttachedToCanvas;
 #endif
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif