add SkLerpXfermode

include/effects/SkLerpXfermode.h

+/*
+ * Copyright 2013 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkLerpXfermode_DEFINED
+#define SkLerpXfermode_DEFINED
+
+#include "SkXfermode.h"
+
+class SK_API SkLerpXfermode : public SkXfermode {
+public:
+    /**
+     *  result = scale * src + (1 - scale) * dst
+     *
+     *  When scale == 1, this is the same as kSrc_Mode
+     *  When scale == 0, this is the same as kDst_Mode
+     */
+    static SkXfermode* Create(SkScalar scale);
+
+    // overrides from SkXfermode
+    virtual void xfer32(SkPMColor dst[], const SkPMColor src[], int count,
+                        const SkAlpha aa[]) const SK_OVERRIDE;
+    virtual void xfer16(uint16_t dst[], const SkPMColor src[], int count,
+                        const SkAlpha aa[]) const SK_OVERRIDE;
+    virtual void xferA8(SkAlpha dst[], const SkPMColor src[], int count,
+                        const SkAlpha aa[]) const SK_OVERRIDE;
+
+    SK_DEVELOPER_TO_STRING()
+    SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkLerpXfermode)
+
+protected:
+    SkLerpXfermode(SkFlattenableReadBuffer&);
+    virtual void flatten(SkFlattenableWriteBuffer&) const SK_OVERRIDE;
+
+private:
+    SkLerpXfermode(unsigned scale256);
+
+    unsigned fScale256;  // 0..256
+
+    typedef SkXfermode INHERITED;
+};
+
+#endif