initialize offscreen in StrokeZeroGM

gm/path_stroke_with_zero_length.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkPath.h"
 #include "SkStream.h"
 #include "gm.h"
@@ skipping 99 matching lines @@
             SkPaint round(butt);
             round.setStrokeCap(SkPaint::kRound_Cap);
             drawPaths(round, 1 << 7);
             return;
         }
 
         SkScalar widths[] = { 0, .999f, 1, 1.001f, 20 };
         bool aliases[] = { false, true };
         for (bool antialias : aliases) {
             canvas->save();
-            for (SkScalar width : widths) { 
+            for (SkScalar width : widths) {
                 canvas->save();
                 SkPaint butt;
                 butt.setAntiAlias(antialias);
                 butt.setStyle(SkPaint::kStroke_Style);
                 butt.setStrokeWidth(width);
                 drawPaths(butt, -1);
 
                 SkPaint round(butt);
                 round.setStrokeCap(SkPaint::kRound_Cap);
                 drawPaths(round, -1);
 
                 SkPaint square(butt);
                 square.setStrokeCap(SkPaint::kSquare_Cap);
                 drawPaths(square, -1);
                 canvas->restore();
                 canvas->translate(220, 0);
             }
             canvas->restore();
             canvas->translate(0, 210);
         }
     }
 
 private:
     void drawFat(SkCanvas* canvas, const SkPath& path, const SkPaint& paint, int index) {
         const SkScalar scale = 10;
         SkRect bounds = path.getBounds();
         SkBitmap offscreen;
         offscreen.allocN32Pixels(SkScalarRoundToInt(bounds.width() + 4),
                 SkScalarRoundToInt(bounds.height() + 4));
+        offscreen.eraseColor(SK_ColorWHITE);
         SkScalar pathX = bounds.fLeft - 2;
         SkScalar pathY = bounds.fTop - 2;
         SkMatrix cMatrix = canvas->getTotalMatrix();
         if (!canvas->readPixels(&offscreen, SkScalarRoundToInt(pathX + cMatrix.getTranslateX()),
                 SkScalarRoundToInt(pathY + cMatrix.getTranslateY()))) {
             return;
         }
 
         canvas->save();
         SkMatrix clipM;
@@ skipping 26 matching lines @@
             canvas->restore();
         }
     }
 
 };
 
 ///////////////////////////////////////////////////////////////////////////////
 
 DEF_GM( return new StrokeZeroGM(); )