Add a GM that reproduces layout test failures with my new xfermode code.

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);
+    }
+}