Fix bug & add code to catch ImageFilter clearing bugs

src/core/SkSpecialSurface.cpp

Index: src/core/SkSpecialSurface.cpp
diff --git a/src/core/SkSpecialSurface.cpp b/src/core/SkSpecialSurface.cpp
index 335e206a6af374072762a11b96a6a79fea1f9f93..dee9811f95098f6d5c35e7275f2897c5dc7f7e31 100644
--- a/src/core/SkSpecialSurface.cpp
+++ b/src/core/SkSpecialSurface.cpp
@@ -74,6 +74,9 @@ public:
 
         fCanvas.reset(new SkCanvas(fBitmap, this->props()));
         fCanvas->clipRect(SkRect::Make(subset));
+#ifdef SK_IS_BOT
+        fCanvas->clear(SK_ColorRED);  // catch any imageFilter sloppiness
+#endif
     }
 
     ~SkSpecialSurface_Raster() override { }
@@ -130,6 +133,9 @@ public:
 
         fCanvas.reset(new SkCanvas(device.get()));
         fCanvas->clipRect(SkRect::Make(subset));
+#ifdef SK_IS_BOT
+        fCanvas->clear(SK_ColorRED);  // catch any imageFilter sloppiness
+#endif
     }
 
     ~SkSpecialSurface_Gpu() override { }