Replace buggy_blend_modes GM with an exhaustive test.

gm/buggy_blend_modes.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 "SkCanvas.h"
-#include "SkGradientShader.h"
-#include "gm.h"
-
-// This GM reproduces what I think are overflow bugs in the CPU implementations of a
-// couple xfermodes.  I've marked non-obvious keys to reproducing the bug // Essential!
-DEF_SIMPLE_GM(buggy_blend_modes, canvas, 800, 200) {
-    const auto tiling = SkShader::kClamp_TileMode;
-    const auto flags = SkGradientShader::kInterpolateColorsInPremul_Flag;  // Essential!
-
-    SkAutoTUnref<SkShader> cyanH, magentaV;
-    {
-        SkPoint    pts[] = { {0,0}, {200, 0} };
-        SkColor colors[] = { 0x00000000, 0xFF00FFFF };
-        cyanH.reset(
-                SkGradientShader::CreateLinear(pts, colors, nullptr, 2, tiling, flags, nullptr));
-    }
-    {
-        SkPoint    pts[] = { {0,0}, {0, 200} };
-        SkColor colors[] = { 0x00000000, 0xFFFF00FF };
-        magentaV.reset(
-                SkGradientShader::CreateLinear(pts, colors, nullptr, 2, tiling, flags, nullptr));
-    }
-
-    SkXfermode::Mode modes[] = {
-        SkXfermode::kDarken_Mode,       // Looks ok?
-        SkXfermode::kHardLight_Mode,    // Definitely wrong.
-        SkXfermode::kLighten_Mode,      // Definitely wrong.
-        SkXfermode::kOverlay_Mode,      // Same code as kHardLight_Mode.
-    };
-
-    canvas->clear(SK_ColorWHITE);
-    for (auto mode : modes) {
-        canvas->saveLayer(nullptr, nullptr);  // Essential!
-            SkPaint h, v;
-            h.setShader(cyanH);
-            v.setShader(magentaV);
-            v.setXfermodeMode(mode);
-            canvas->drawRect(SkRect::MakeWH(200,200), h);
-            canvas->drawRect(SkRect::MakeWH(200,200), v);
-        canvas->restore();
-
-        canvas->translate(200, 0);
-    }
-}