Add an avoidXfermode GM to exercise coverage

gm/avoidxfermode3.cpp

Index: gm/avoidxfermode3.cpp
diff --git a/gm/avoidxfermode3.cpp b/gm/avoidxfermode3.cpp
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+++ b/gm/avoidxfermode3.cpp
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+/*
+* Copyright 2016 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"
+
+#include "SkAvoidXfermode.h"
+
+// This GM exercises how the avoid xfer mode interacts with partial coverage
+class AvoidXfermode3GM : public skiagm::GM {
+public:
+    AvoidXfermode3GM() { }
+
+protected:
+    SkString onShortName() override {
+        return SkString("avoidxfermode3");
+    }
+
+    SkISize onISize() override { return SkISize::Make(128, 128); }
+
+    void onDraw(SkCanvas* canvas) override {
+        canvas->drawARGB(255, 0, 0, 0);
+
+        SkRect r = SkRect::MakeXYWH(16.5f, 16.5f, 96.0f, 96.0f);
+
+        SkPaint p0;
+        p0.setColor(SK_ColorWHITE);
+        p0.setAntiAlias(true);
+
+        canvas->drawRect(r, p0);
+
+        r = SkRect::MakeXYWH(3.5f, 3.5f, 59.0f, 59.0f);
+
+        // UL corner: replace white with green with a tight tolerance
+        SkPaint p1;
+        p1.setColor(SK_ColorGREEN);
+        p1.setAntiAlias(true);
+        p1.setXfermode(SkAvoidXfermode::Create(SK_ColorWHITE,
+                                               5,
+                                               SkAvoidXfermode::kTargetColor_Mode))->unref();
+
+        canvas->drawRect(r, p1);
+
+        r.offsetTo(65.5f, 3.5f);
+
+        // UR corner: draw red everywhere except white with a tight tolerance
+        SkPaint p2;
+        p2.setColor(SK_ColorRED);
+        p2.setAntiAlias(true);
+        p2.setXfermode(SkAvoidXfermode::Create(SK_ColorWHITE,
+                                               250,
+                                               SkAvoidXfermode::kAvoidColor_Mode))->unref();
+
+        canvas->drawRect(r, p2);
+
+        r.offsetTo(3.5f, 65.5f);
+
+        // LL corner: replace white with blue with a loose tolerance
+        SkPaint p3;
+        p3.setColor(SK_ColorBLUE);
+        p3.setAntiAlias(true);
+        p3.setXfermode(SkAvoidXfermode::Create(SK_ColorWHITE,
+                                               250,
+                                               SkAvoidXfermode::kTargetColor_Mode))->unref();
+
+        canvas->drawRect(r, p3);
+
+        r.offsetTo(65.5f, 65.5f);
+
+        // LR corner: draw yellow everywhere except white with a loose tolerance
+        SkPaint p4;
+        p4.setColor(SK_ColorYELLOW);
+        p4.setAntiAlias(true);
+        p4.setXfermode(SkAvoidXfermode::Create(SK_ColorWHITE,
+                                               5,
+                                               SkAvoidXfermode::kAvoidColor_Mode))->unref();
+
+        canvas->drawRect(r, p4);
+    }
+
+private:
+    typedef GM INHERITED;
+};
+
+//////////////////////////////////////////////////////////////////////////////
+
+DEF_GM(return new AvoidXfermode3GM;)