Use ARM fixes for SDF encoding.

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)
+// 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
 #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.
+// For the fragment shader:
+//   The distance field is constructed as unsigned char values,
+//   so that the zero value is at 128, and the supported range of distances is [-4 * 127/128, 4].
+//   Hence our multiplier (width of the range) is 4 * 255/128 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
  *                           by the client with the padding above.
  *  @param image             8-bit mask we're using to generate the distance field.
  *  @param w                 Width of the original image.
  *  @param h                 Height of the original image.
@@ skipping 18 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