Add AlphaThreshold filter.

include/effects/SkAlphaThresholdFilter.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 SkAlphaThresholdFilter_DEFINED
+#define SkAlphaThresholdFilter_DEFINED
+
+#include "SkRegion.h"
+#include "SkImageFilter.h"
+
+class SK_API SkAlphaThresholdFilter {
+public:
+    /**
+     * Creates an image filter that samples a region. If the sample is inside the
+     * region the alpha of the image is boosted up to a threshold value. If it is
+     * outside the region then the alpha is decreased to the threshold value.
+     * The 0,0 point of the region corresponds to the upper left corner of the
+     * source image.
+     */
+    static SkImageFilter* Create(const SkRegion& region, SkScalar innerThreshold, SkScalar outerThreshold);
+};
+
+#endif