Factor out checkerboard function in gm and sampleapp into tools.

gm/transparency.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 "SkGradientShader.h"
+#include "SkSurface.h"
+#include "checkerboard.h"
+#include "gm.h"
+
+static void make_transparency(SkCanvas* canvas,
+                              SkScalar width,
+                              SkScalar height) {
+    SkPoint pts[2];
+    pts[0] = SkPoint::Make(0, 0);
+    pts[1] = SkPoint::Make(width, 0);
+    const SkColor kColors[] = {
+            SK_ColorBLACK,
+            SK_ColorGRAY,
+            SK_ColorWHITE,
+            SK_ColorRED,
+            SK_ColorYELLOW,
+            SK_ColorGREEN,
+            SK_ColorCYAN,
+            SK_ColorBLUE,
+            SK_ColorMAGENTA,
+    };
+    const SkScalar kRowHeight = height / SK_ARRAY_COUNT(kColors);
+    for (size_t i = 0; i < SK_ARRAY_COUNT(kColors); ++i) {
+        SkColor shaderColors[2];
+        shaderColors[0] = SK_AlphaTRANSPARENT;
+        shaderColors[1] = kColors[i];
+        SkAutoTUnref<SkShader> shader(SkGradientShader::CreateLinear(
+                pts, shaderColors, NULL, 2, SkShader::kClamp_TileMode));
+        SkRect r = SkRect::MakeXYWH(0, i * kRowHeight, width, kRowHeight);
+        SkPaint p;
+        p.setShader(shader);
+        canvas->drawRect(r, p);
+    }
+}
+
+/**
+ *  This GM verifies that a transparent bitmap drawn over a
+ *  checkerboard pattern looks correct.
+ */
+DEF_SIMPLE_GM(transparency_check, canvas, 1912, 1080) {
+    skiagm::Checkerboard(canvas);
+    {
+        SkAutoCanvasRestore autoCanvasRestore(canvas, true);
+        SkAutoTUnref<SkSurface> surface(SkSurface::NewRasterN32Premul(256, 9));
+        make_transparency(surface->getCanvas(), 256.0f, 9.0f);
+        canvas->scale(7.5f, 120.0f);
+        surface->draw(canvas, 0, 0, NULL);
+    }
+}