antialias matrix-image-filter to get smooth diagonals

src/core/SkDeviceImageFilterProxy.h

 /*
  * Copyright 2012 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 SkDeviceImageFilterProxy_DEFINED
 #define SkDeviceImageFilterProxy_DEFINED
 
 #include "SkBitmapDevice.h"
 #include "SkDevice.h"
 #include "SkImageFilter.h"
 #include "SkSurfaceProps.h"
 
 class SkDeviceImageFilterProxy : public SkImageFilter::Proxy {
 public:
     SkDeviceImageFilterProxy(SkBaseDevice* device, const SkSurfaceProps& props)
         : fDevice(device)
         , fProps(props.flags(), kUnknown_SkPixelGeometry)
     {}
 
     SkBaseDevice* createDevice(int w, int h) override {
+        // Our contract is to return a zero-initialized device, so we request iPremul_AlphaType
+        // which is the signal to the fDevice to perform this init.
         SkBaseDevice::CreateInfo cinfo(SkImageInfo::MakeN32Premul(w, h),
                                        SkBaseDevice::kNever_TileUsage,
                                        kUnknown_SkPixelGeometry,
                                        true /*forImageFilter*/);
         SkBaseDevice* dev = fDevice->onCreateDevice(cinfo, NULL);
         if (NULL == dev) {
             dev = SkBitmapDevice::Create(cinfo.fInfo);
         }
         return dev;
     }
     bool canHandleImageFilter(const SkImageFilter* filter) override {
         return fDevice->canHandleImageFilter(filter);
     }
     virtual bool filterImage(const SkImageFilter* filter, const SkBitmap& src,
                              const SkImageFilter::Context& ctx,
                              SkBitmap* result, SkIPoint* offset) override {
         return fDevice->filterImage(filter, src, ctx, result, offset);
     }
 
     const SkSurfaceProps* surfaceProps() const override {
         return &fProps;
     }
 
 private:
     SkBaseDevice*  fDevice;
     const SkSurfaceProps fProps;
 };
 
 #endif