handle halfway case in scan converter

gm/composeshader.cpp

Index: gm/composeshader.cpp
diff --git a/gm/composeshader.cpp b/gm/composeshader.cpp
index e012bbb28590b21282b59b47a3da5173c4e8603b..b2be9ba28e463b7aba3b22d053406e5d835ef28e 100644
--- a/gm/composeshader.cpp
+++ b/gm/composeshader.cpp
@@ -162,7 +162,14 @@ static SkShader* make_linear_gradient_shader(int length) {
 
 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;
@@ -173,6 +180,7 @@ public:
                                                      SkShader::kRepeat_TileMode, &s);
         fLinearGradientShader = make_linear_gradient_shader(squareLength);
     }
+
     ~ComposeShaderBitmapGM() {
         SkSafeUnref(fColorBitmapShader);
         SkSafeUnref(fAlpha8BitmapShader);