Fix bug & add code to catch ImageFilter clearing bugs

src/effects/SkXfermodeImageFilter.cpp

Index: src/effects/SkXfermodeImageFilter.cpp
diff --git a/src/effects/SkXfermodeImageFilter.cpp b/src/effects/SkXfermodeImageFilter.cpp
index 99ce24f1295b8c80818a62d54663c92e69fad1a2..d4756e8e788305c9ba58235d0b6b0069339bf76c 100644
--- a/src/effects/SkXfermodeImageFilter.cpp
+++ b/src/effects/SkXfermodeImageFilter.cpp
@@ -104,6 +104,8 @@ sk_sp<SkSpecialImage> SkXfermodeImageFilter::onFilterImage(SkSpecialImage* sourc
     SkCanvas* canvas = surf->getCanvas();
     SkASSERT(canvas);
 
+    canvas->clear(0x0); // can't count on background to fully clear the background
+
     canvas->translate(SkIntToScalar(-bounds.left()), SkIntToScalar(-bounds.top()));
 
     SkPaint paint;