Add PlusMergesAA GM

gm/plus.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 "gm.h"
+
+DEF_SIMPLE_GM(PlusMergesAA, canvas, 256, 256) {
+    SkPaint p;
+    p.setColor(SK_ColorRED);
+    p.setAntiAlias(true);  //  <-- crucial to the test that we use AA
+
+    // Draw a two red squares.
+    canvas->drawRect(SkRect::MakeWH(100, 100), p);
+    canvas->drawRect(SkRect::MakeXYWH(150, 0, 100, 100), p);
+
+    p.setColor(0xf000ff00);
+
+    // We'll draw a green square on top of each using two triangles.
+    SkPath upperLeft;
+    upperLeft.lineTo(100, 0);
+    upperLeft.lineTo(0, 100);
+    upperLeft.lineTo(0, 0);
+
+    SkPath bottomRight;
+    bottomRight.moveTo(100, 0);
+    bottomRight.lineTo(100, 100);
+    bottomRight.lineTo(0, 100);
+    bottomRight.lineTo(100, 0);
+
+    // The left square is drawn simply with SrcOver.  It will show a red seam.
+    canvas->drawPath(upperLeft, p);
+    canvas->drawPath(bottomRight, p);
+
+    // Using Plus on the right should merge the AA of seam together completely covering the red.
+    canvas->saveLayer(nullptr, nullptr);
+      p.setXfermodeMode(SkXfermode::kPlus_Mode);
+      canvas->translate(150, 0);
+      canvas->drawPath(upperLeft, p);
+      canvas->drawPath(bottomRight, p);
+    canvas->restore();
+}