Add explicit clip to SkSpecialSurface's SkCanvas

src/core/SkSpecialSurface.cpp

Index: src/core/SkSpecialSurface.cpp
diff --git a/src/core/SkSpecialSurface.cpp b/src/core/SkSpecialSurface.cpp
index 83a0ee7a19cec041198f7e5ea0974af7a2b40122..eecdaecd612b2c81be413d745f646f053f3b21c8 100644
--- a/src/core/SkSpecialSurface.cpp
+++ b/src/core/SkSpecialSurface.cpp
@@ -77,6 +77,7 @@ public:
         fBitmap.setPixelRef(pr);
 
         fCanvas.reset(new SkCanvas(fBitmap, this->props()));
+        fCanvas->clipRect(SkRect::Make(subset));
     }
 
     ~SkSpecialSurface_Raster() override { }
@@ -134,6 +135,7 @@ public:
         }
 
         fCanvas.reset(new SkCanvas(device));
+        fCanvas->clipRect(SkRect::Make(subset));
     }
 
     ~SkSpecialSurface_Gpu() override { }