Move SkAvoidXfermode over from Android

include/effects/SkAvoidXfermode.h

+/*
+ * Copyright 2006 The Android Open Source Project
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkAvoidXfermode_DEFINED
+#define SkAvoidXfermode_DEFINED
+
+#include "SkColor.h"
+#include "SkTypes.h"
+#include "SkXfermode.h"
+
+/** \class AvoidXfermode
+
+    This xfermode will draw the src everywhere except on top of the specified
+    color.
+*/
+class SkAvoidXfermode : public SkXfermode {
+public:
+    enum Mode {
+        kAvoidColor_Mode,   //!< draw everywhere except on the opColor
+        kTargetColor_Mode   //!< draw only on top of the opColor
+    };
+
+    /** This xfermode draws, or doesn't draw, based on the destination's
+        distance from an op-color.
+
+        There are two modes, and each mode interprets a tolerance value.
+
+        Avoid: In this mode, drawing is allowed only on destination pixels that
+               are different from the op-color.
+               Tolerance near 0: avoid any colors even remotely similar to the op-color
+               Tolerance near 255: avoid only colors nearly identical to the op-color
+
+        Target: In this mode, drawing only occurs on destination pixels that
+                are similar to the op-color
+                Tolerance near 0: draw only on colors that are nearly identical to the op-color
+                Tolerance near 255: draw on any colors even remotely similar to the op-color
+     */
+    static SkAvoidXfermode* Create(SkColor opColor, U8CPU tolerance, Mode mode) {
+        return new SkAvoidXfermode(opColor, tolerance, mode);
+    }
+
+    // overrides from SkXfermode
+    void xfer32(SkPMColor dst[], const SkPMColor src[], int count,
+                const SkAlpha aa[]) const override;
+    void xfer16(uint16_t dst[], const SkPMColor src[], int count,
+                const SkAlpha aa[]) const override;
+    void xferA8(SkAlpha dst[], const SkPMColor src[], int count,
+                const SkAlpha aa[]) const override;
+
+    SK_TO_STRING_OVERRIDE()
+    SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(AvoidXfermode)
+
+protected:
+    SkAvoidXfermode(SkColor opColor, U8CPU tolerance, Mode mode);
+    void flatten(SkWriteBuffer&) const override;
+
+private:
+    SkColor     fOpColor;
+    uint32_t    fDistMul;   // x.14 cached from fTolerance
+    uint8_t     fTolerance;
+    Mode        fMode;
+
+    typedef SkXfermode INHERITED;
+};
+
+#endif