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.

[Joel.Liang, 2016/02/24 04:10:23]

The range here is wrong. It should be [-4, 4 * 127/128) or (-4 * 127/128, 4].
And we can also change the multiplier to 4 + 4 * 127/128 or 8 * 255/256 for
better understanding. (The result value is the same as 8 - 1/32)

[jvanverth1, 2016/02/24 14:57:12]

Fixed.

 #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