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..be0158c3c9849f475839050c339821609663bfaa 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_DEBUG
+        fCanvas->clear(SK_ColorRED);  // catch any imageFilter sloppiness

[Stephen White, 2016/05/12 15:43:58]

This is a good idea, but I'm not sure we should do this in Debug, since it'll
change behaviour. Are there other areas in GPU where we have different pixel
results in Debug to detect errors? Maybe keep this behind a different #ifdef?

[robertphillips, 2016/05/12 17:00:57]

Done. Now enabled by SK_IS_BOT.

+#endif
     }
 
     ~SkSpecialSurface_Raster() override { }
@@ -130,6 +133,9 @@ public:
 
         fCanvas.reset(new SkCanvas(device.get()));
         fCanvas->clipRect(SkRect::Make(subset));
+#ifdef SK_DEBUG
+        fCanvas->clear(SK_ColorRED);  // catch any imageFilter sloppiness
+#endif
     }
 
     ~SkSpecialSurface_Gpu() override { }