Fix bug & add code to catch ImageFilter clearing bugs

src/effects/SkXfermodeImageFilter.cpp

 /*
  * Copyright 2013 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkXfermodeImageFilter.h"
 
 #include "SkCanvas.h"
@@ skipping 86 matching lines @@
     const SkImageInfo info = SkImageInfo::MakeN32(bounds.width(), bounds.height(),
                                                   kPremul_SkAlphaType);
     sk_sp<SkSpecialSurface> surf(source->makeSurface(info));
     if (!surf) {
         return nullptr;
     }
 
     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;
     paint.setXfermodeMode(SkXfermode::kSrc_Mode);
 
     if (background) {
         background->draw(canvas,
                          SkIntToScalar(backgroundOffset.fX), SkIntToScalar(backgroundOffset.fY),
                          &paint);
     }
@@ skipping 131 matching lines @@
     SkMatrix matrix;
     matrix.setTranslate(SkIntToScalar(-bounds.left()), SkIntToScalar(-bounds.top()));
     drawContext->drawRect(GrClip::WideOpen(), paint, matrix, SkRect::Make(bounds));
 
     return SkSpecialImage::MakeFromGpu(SkIRect::MakeWH(bounds.width(), bounds.height()),
                                        kNeedNewImageUniqueID_SpecialImage,
                                        drawContext->asTexture());
 }
 
 #endif