Antialias matrix-image-filter to get smooth diagonals

src/core/SkDeviceImageFilterProxy.h

Index: src/core/SkDeviceImageFilterProxy.h
diff --git a/src/core/SkDeviceImageFilterProxy.h b/src/core/SkDeviceImageFilterProxy.h
index 79ff957564c48af5661dff3e3e5d0296499e2070..d77f3f62cdd30433cf642cc93d047e1b0fea8e5a 100644
--- a/src/core/SkDeviceImageFilterProxy.h
+++ b/src/core/SkDeviceImageFilterProxy.h
@@ -21,6 +21,8 @@ public:
     {}
 
     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,