Use ARM fixes for SDF encoding.
This pulls out the encoding fixes by Joel Liang from
https://codereview.chromium.org/1643143002/
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1726763002
Review URL: https://codereview.chromium.org/1726763002
diff --git a/src/core/SkDistanceFieldGen.h b/src/core/SkDistanceFieldGen.h
index 4a09ace..5e7af52 100644
--- a/src/core/SkDistanceFieldGen.h
+++ b/src/core/SkDistanceFieldGen.h
@@ -10,7 +10,7 @@
#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
@@ -18,9 +18,10 @@
// 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"