handle halfway case in scan converter

gm/composeshader.cpp

 /*
  * Copyright 2012 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 "SkBitmapProcShader.h"
@@ skipping 144 matching lines @@
     pts[0].set(0, 0);
     pts[1].set(SkIntToScalar(length), 0);
     colors[0] = SK_ColorBLUE;
     colors[1] = SkColorSetARGB(0, 0, 0, 0xFF);
     return SkGradientShader::CreateLinear(pts, colors, nullptr, 2, SkShader::kClamp_TileMode);
 }
 
 
 class ComposeShaderBitmapGM : public skiagm::GM {
 public:
-    ComposeShaderBitmapGM() {
+    ComposeShaderBitmapGM()
+        : fColorBitmapShader(nullptr)
+        , fAlpha8BitmapShader(nullptr)
+        , fLinearGradientShader(nullptr)
+    {
+    }
+
+    void onOnceBeforeDraw() override {
         draw_color_bm(&fColorBitmap, squareLength);
         draw_alpha8_bm(&fAlpha8Bitmap, squareLength);
         SkMatrix s;
         s.reset();
         fColorBitmapShader = new SkBitmapProcShader(fColorBitmap, SkShader::kRepeat_TileMode,
                                                     SkShader::kRepeat_TileMode, &s);
         fAlpha8BitmapShader = new SkBitmapProcShader(fAlpha8Bitmap, SkShader::kRepeat_TileMode,
                                                      SkShader::kRepeat_TileMode, &s);
         fLinearGradientShader = make_linear_gradient_shader(squareLength);
     }
+
     ~ComposeShaderBitmapGM() {
         SkSafeUnref(fColorBitmapShader);
         SkSafeUnref(fAlpha8BitmapShader);
         SkSafeUnref(fLinearGradientShader);
     }
 
 protected:
     SkString onShortName() override {
         return SkString("composeshader_bitmap");
     }
@@ skipping 51 matching lines @@
     SkShader* fLinearGradientShader;
 
     typedef GM INHERITED;
 };
 
 //////////////////////////////////////////////////////////////////////////////
 
 DEF_GM( return new ComposeShaderGM; )
 DEF_GM( return new ComposeShaderAlphaGM; )
 DEF_GM( return new ComposeShaderBitmapGM; )