Add new module for distance field generation.

src/core/SkDistanceFieldGen.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.
+ */
+#ifndef SkDistanceFieldGen_DEFINED
+#define SkDistanceFieldGen_DEFINED
+
+/** Given 8-bit mask data, generate the associated distance field
+
+ *  @param distanceField     The distance field to be generated. Should already be allocated
+ *                           by the client with the padding below.
+ *  @param image             8-bit mask we're using to generate the distance field.
+ *  @param w                 Width of the image.
+ *  @param h                 Height of the image.
+ *  @param distanceMagnitude Largest possible absolute value for the distance. The distance field
+ *                           will be padded to w + 2*distanceMagnitude, h + 2*distanceMagnitude.
+ */
+bool SkGenerateDistanceFieldFromImage(unsigned char* distanceField,
+                                      const unsigned char* image,
+                                      int w, int h,
+                                      int distanceMagnitude);
+
+#endif