Fix bug & add code to catch ImageFilter clearing bugs

src/core/SkSpecialSurface.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file
  */
 
 #include "SkCanvas.h"
 #include "SkSpecialImage.h"
 #include "SkSpecialSurface.h"
@@ skipping 56 matching lines @@
                             const SkIRect& subset,
                             const SkSurfaceProps* props)
         : INHERITED(subset, props) {
         const SkImageInfo& info = pr->info();
 
         fBitmap.setInfo(info, info.minRowBytes());
         fBitmap.setPixelRef(pr);
 
         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 { }
 
     sk_sp<SkSpecialImage> onMakeImageSnapshot() override {
         return SkSpecialImage::MakeFromRaster(this->subset(), fBitmap, &this->props());
     }
 
 private:
     SkBitmap fBitmap;
@@ skipping 36 matching lines @@
 
         sk_sp<SkGpuDevice> device(SkGpuDevice::Make(sk_ref_sp(fTexture->asRenderTarget()),
                                                     width, height, props,
                                                     SkGpuDevice::kUninit_InitContents));
         if (!device) {
             return;
         }
 
         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 { }
 
     sk_sp<SkSpecialImage> onMakeImageSnapshot() override {
         // Note: we are intentionally zeroing out 'fTexture' here
         return SkSpecialImage::MakeFromGpu(this->subset(),
                                            kNeedNewImageUniqueID_SpecialImage,
                                            std::move(fTexture),
                                            &this->props());
@@ skipping 22 matching lines @@
     if (!tex) {
         return nullptr;
     }
 
     const SkIRect subset = SkIRect::MakeWH(width, height);
 
     return sk_make_sp<SkSpecialSurface_Gpu>(std::move(tex), width, height, subset, nullptr);
 }
 
 #endif