Fix artifacts in distance field rendering due to bilerp

src/core/SkDistanceFieldGen.h

Index: src/core/SkDistanceFieldGen.h
diff --git a/src/core/SkDistanceFieldGen.h b/src/core/SkDistanceFieldGen.h
index 756cff14a3be2fd38778578dd48ee5c2d9b62e12..0510907f8c8847ec319f4c8fd037d39720542f6f 100644
--- a/src/core/SkDistanceFieldGen.h
+++ b/src/core/SkDistanceFieldGen.h
@@ -14,8 +14,9 @@
 #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,