Fix artifacts in distance field rendering due to bilerp

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
 
 #include "SkTypes.h"
 
 // the max magnitude for the distance field
 // distance values are limited to the range [-SK_DistanceFieldMagnitude, SK_DistanceFieldMagnitude)
 #define SK_DistanceFieldMagnitude   4
 // we need to pad around the original glyph to allow our maximum distance of
 // SK_DistanceFieldMagnitude texels away from any edge
-// we add one to this pad to allow for bilerp
-#define SK_DistanceFieldPad     (SK_DistanceFieldMagnitude+1)
+#define SK_DistanceFieldPad         4
+// the rect we render with is inset from the distance field glyph size to allow for bilerp
+#define SK_DistanceFieldInset       2
 
 // for the fragment shader
 // The distance field is constructed as unsigned char values, so that the zero value is at 128,
 // and the range is [-4, 4 - 1/255). Hence our multiplier is 8 - 1/32 and zero threshold is 128/255.
 #define SK_DistanceFieldMultiplier   "7.96875"
 #define SK_DistanceFieldThreshold    "0.50196078431"
 
 /** Given 8-bit mask data, generate the associated distance field
 
  *  @param distanceField     The distance field to be generated. Should already be allocated
@@ skipping 22 matching lines @@
 
 /** Given width and height of original image, return size (in bytes) of distance field
  *  @param w                 Width of the original image.
  *  @param h                 Height of the original image.
  */
 inline size_t SkComputeDistanceFieldSize(int w, int h) {
     return (w + 2*SK_DistanceFieldPad) * (h + 2*SK_DistanceFieldPad) * sizeof(unsigned char);
 }
 
 #endif