add maskfilter to c api

include/c/sk_maskfilter.h

+/*
+ * Copyright 2014 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+// EXPERIMENTAL EXPERIMENTAL EXPERIMENTAL EXPERIMENTAL
+// DO NOT USE -- FOR INTERNAL TESTING ONLY
+
+#ifndef sk_maskfilter_DEFINED
+#define sk_maskfilter_DEFINED
+
+#include "sk_types.h"
+
+typedef enum {
+    NORMAL_SK_BLUR_STYLE,   //!< fuzzy inside and outside
+    SOLID_SK_BLUR_STYLE,    //!< solid inside, fuzzy outside
+    OUTER_SK_BLUR_STYLE,    //!< nothing inside, fuzzy outside
+    INNER_SK_BLUR_STYLE,    //!< fuzzy inside, nothing outside
+} sk_blurstyle_t;
+
+SK_C_PLUS_PLUS_BEGIN_GUARD
+
+void sk_maskfilter_ref(sk_maskfilter_t*);
+void sk_maskfilter_unref(sk_maskfilter_t*);
+
+sk_maskfilter_t* sk_maskfilter_new_blur(sk_blurstyle_t, float sigma);
+
+SK_C_PLUS_PLUS_END_GUARD
+
+#endif