Fix bug & add code to catch ImageFilter clearing bugs

gm/pictureimagefilter.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "gm.h"
 
 #include "SkPictureImageFilter.h"
 #include "SkPictureRecorder.h"
 
 // This GM exercises the SkPictureImageFilter ImageFilter class.
 
 static void fill_rect_filtered(SkCanvas* canvas,
                                const SkRect& clipRect,
                                sk_sp<SkImageFilter> filter) {
     SkPaint paint;
     paint.setImageFilter(filter);
     canvas->save();
     canvas->clipRect(clipRect);
     canvas->drawPaint(paint);
     canvas->restore();
 }
 
 static sk_sp<SkPicture> make_picture() {
     SkPictureRecorder recorder;
     SkCanvas* canvas = recorder.beginRecording(100, 100, nullptr, 0);
-    canvas->clear(SK_ColorBLACK);
     SkPaint paint;
     paint.setAntiAlias(true);
     sk_tool_utils::set_portable_typeface(&paint);
     paint.setColor(0xFFFFFFFF);
     paint.setTextSize(SkIntToScalar(96));
     const char* str = "e";
     canvas->drawText(str, strlen(str), SkIntToScalar(20), SkIntToScalar(70), paint);
     return recorder.finishRecordingAsPicture();
 }
 
@@ skipping 104 matching lines @@
 private:
     sk_sp<SkPicture> fPicture;
     sk_sp<SkPicture> fLCDPicture;
 
     typedef GM INHERITED;
 };
 
 ///////////////////////////////////////////////////////////////////////////////
 
 DEF_GM( return new PictureImageFilterGM; )